CORBIS, FRANS LANTING
Pitch dark, dank, and seething with saber-toothed, sausage-shaped creatures, the world of the African naked mole-rat is a hostile habitat. In the 1980s, scientists made the remarkable discovery that naked mole-rats live like termites with a single, dominant breeding queen and scores of nonbreeding adult helpers that never leave their natal colony. But the bizarreness doesn’t stop there. Naked mole-rats, unlike other mammals, tolerate variable body temperatures, attributed to their lack of an insulatory layer of fur. Their pink skin is hairless except for sparse, whisker-like strands that crisscross the body to form a sensitive sensory array that helps them navigate in the dark. Both the naked mole-rat’s skin and its upper respiratory tract are completely insensitive to chemical irritants such as acids and capsaicin, the spicy ingredient in chili peppers. Most surprisingly, they can survive periods of oxygen deprivation that would cause irreversible brain damage...
Brain tissue of naked mole-rats remains functional with no oxygen supply for more than three times as long as brain tissue of laboratory mice.
The current hypotheses for the existence of this suite of unusual features center around the equally unusual lifestyle traits of the naked mole-rat. (See illustration on page 33.) Naked mole-rats live in large family groups in elaborate underground burrows. Although they are protected from large temperature fluctuations as well as from predators and pathogens, they have to contend with low oxygen and high carbon dioxide levels, due to the large number of individuals—usually 100 to 300—living and respiring in close quarters under poorly ventilated conditions. The unusual ecology and social structure of the naked mole-rat make this an exciting system for understanding evolution and specialization, and details of the molecular mechanisms underlying the mole-rat’s unusually good health are providing insights into human disease.
No oxygen? No problem!
Most mammalian brains, including those of humans, start to suffer damage after just 3–4 minutes of oxygen deprivation. This is because brain tissue does not store much energy, and a steady supply of oxygen is needed to generate more. Hence, when the oxygen supply to the brain is reduced or blocked, brain cells run out of energy, and damage quickly ensues. This is a major concern for victims of heart attacks and strokes, in which the blood supply to the brain is interrupted. Brain tissue of naked mole-rats, on the other hand, remains functional with no oxygen supply for more than three times as long as brain tissue of laboratory mice. And when the oxygen level is restored, brain tissue from naked mole-rats frequently recovers fully, even after several minutes of inactivity.[1. J. Larson, T.J. Park, “Extreme hypoxia tolerance of naked mole-rat brain,” NeuroReport, 20:1634-37, 2009.]
This remarkable ability no doubt stems from the challenge that all subterranean animals face: low oxygen levels because of poor air exchange with the surface. Oxygen depletion is even more pronounced for naked mole-rats because they live in large groups, with many individuals sharing the same poor air supply, and gas exchange is limited to diffusion or air turbulence caused by animals moving in the tunnels. So how do mole-rats survive in such smothering conditions?
Naked mole-rats display several physiological adaptations for survival in a low-oxygen environment. The hemoglobin in their red blood cells has a higher affinity for oxygen than that of most other mammals, meaning that their blood is better at capturing what little oxygen there is. They also have a greater number of red blood cells per unit volume. In addition, their mass-specific metabolic rate is only about 70 percent that of other rodents, so they use oxygen at a slower rate. But when it comes to the brain, naked mole-rats protect themselves by borrowing a strategy used by the brains of infants.
Infant mammals, including humans, are known to be much more tolerant of oxygen deprivation than older juveniles or adults. It turns out that calcium is a key factor in this tolerance. Normally, calcium ions in our brain cells play vital roles, including helping memories form. But it’s a delicate balance: small amounts of calcium are essential for brain function, but too much calcium makes things go haywire. When nerve cells are starved of oxygen, they no longer have the energy to regulate calcium entry, resulting in an influx of too much calcium, which poisons the cells. This is the primary cause of neuronal death during oxygen deprivation.
In the last decade or so, researchers discovered that adult and infant brains express different calcium channels in their cell membranes. Calcium channels in infants actually close during oxygen deprivation, protecting the brain cells from calcium overdose in the womb, where the baby gets much less oxygen. After the baby is born, however, oxygen is plentiful, and these channels are largely replaced by ones that open in response to oxygen deprivation, often leading to cell death.
Recent studies on naked mole-rats show that this species retains infant-style calcium channels into adulthood.[2. B.L. Peterson et al., “Adult naked mole-rat brain retains the NMDA receptor subunit GluN2D associated with hypoxia tolerance in neonatal mammals,” Neurosci Lett, 506:342-45, 2012.] Accordingly, calcium-imaging techniques show that oxygen deprivation leads to much less calcium entry into the brain cells of adult naked mole-rats compared to other adult mammals.[3. B.L. Peterson et al., “Blunted neuronal calcium response to hypoxia in naked mole-rat hippocampus,” PLoS One, 7:e31568, 2012.] These findings suggest a new strategy that may help human victims of heart attack and stroke: increase the numbers of infant-style calcium channels in the brain. Brain cells of adult humans actually have some of these channels already, just not enough to protect them during oxygen deprivation. If a drug is designed to quickly upregulate production of infant-style channels in the brains of heart attack and stroke victims, it could provide valuable protection during a time when a steady supply of oxygen-rich blood is not reaching the brain.
Feeling no pain
In addition to dealing with low oxygen levels, living in crowded underground burrows also means naked mole-rats must contend with high carbon dioxide (CO2) concentrations. In contrast to the typical atmospheric concentration of CO2 of about 0.03 percent, CO2 levels in naked mole-rat tunnels are closer to 2 percent, possibly reaching concentrations of 5 percent or more in their nest chambers. High levels of CO2 can be painful to the eyes and nose due to the formation of acid on the surface of those tissues—akin to the feeling of burping through one’s nose after drinking a carbonated beverage—but mole-rats are completely insensitive to this phenomenon. The skin and upper respiratory tract of naked mole-rats are also insensitive to other irritants, including other acids, ammonia, and capsaicin. Behaviorally, the animals show no signs of irritation or discomfort when a capsaicin solution is applied to their nostrils, whereas mice vigorously rub their noses after such exposure. Unlike rats and mice, naked mole-rats also fail to avoid strong ammonia fumes. When placed in an arena with sponges that are saturated with ammonia or water, mole-rats spend as much time in close proximity to the ammonia as they do to the water. The animals also show no response to capsaicin or acidic saline (like lemon juice) injected into the skin of the foot, while the same irritants cause rubbing and scratching at the injection site in humans and vigorous licking in rats and mice.
Recent experiments have shown that nerve fibers called C-fibers, which normally respond to high levels of CO2and other chemical irritants, are much less sensitive in naked mole-rats than in other mammals. These fibers are small in diameter, and release neuropeptides—notably Substance P and calcitonin gene-related peptide—onto targets in the central nervous system to convey a stinging or burning sensation. Importantly, the same C-fibers that respond to acid and capsaicin are responsible for the pain people experience minutes, hours, or even days after an injury.
Surprisingly, physiological studies revealed that naked mole-rat C-fibers innervating their eyes, nose, and skin do respond to capsaicin, but that the nerves do not make the neuropeptides usually released because of a defect in gene promoters associated with the pain-relaying nerve cells. While the animals express the neuropeptides in other parts of the body, such as the brain and intestines, lack of these neuropeptides from the C-fibers acts to “disconnect” the fibers from the central nervous system, preventing the feelings of pain and irritation. Sure enough, when researchers introduced one of the missing neuropeptides, Substance P, into the C-fibers of naked mole-rat feet using gene therapy, the animals licked at the injection site similarly to rats and mice.[4. T.J. Park et al., “Selective inflammatory pain insensitivity in the African naked mole-rat (Heterocephalus glaber),” PLoS Biol, 6:e13, 2008.]
Insensitivity to acidic saline appears to be mediated by a different mechanism. In contrast to their response to capsaicin, C-fibers in naked mole-rats are completely unresponsive to acidic saline. A recent study revealed that acid insensitivity involves voltage-gated sodium channels, which are necessary to propagate signals along the nerve fibers.[5. E.S. Smith et al., “The molecular basis of acid insensitivity in the African naked mole-rat,” Science, 334:1557-60, 2011.] In naked mole-rats, these channels have a mutation that make them shut down under acidic conditions.
Naked mole-rat C-fibers also have an unusual pattern of connectivity in the spinal cord. Almost half of the cells in the deep dorsal horn of the spinal cord receive direct connections from C-fibers, whereas in other species, most C-fibers terminate in the superficial dorsal horn, at the outer edge of the spinal cord. The significance of this unusual connection pattern is not clear, but it suggests that whatever signals are conveyed from the C-fibers might not follow the usual pain and irritant pathways once they reach the spinal cord.
Interestingly, naked mole-rats respond normally to pinch and heat; only C fiber-mediated pain has been muted in these animals. A greater understanding of how this type of pain processing is altered in naked mole-rats could have significant implications for the treatment of chronic pain in humans, such as post-surgical, joint and muscle, and inflammatory pain.
Unlike mice, which very commonly develop tumors, naked mole-rats have never been found to naturally have cancer. Moreover, subjecting mole-rats to ionizing radiation does not induce much DNA damage, as seen in other animals, nor does it result in tumors, even 5 years later. Attempts to turn naked mole-rat cells cancerous via injection of oncogenes have also failed, whereas similar methods using human, mouse, and even cattle cells results in conversion to highly aggressive and invasive cancer-forming cells.[6. S. Liang et al., “Resistance to experimental tumorigenesis in cells of a long-lived mammal, the naked mole-rat (Heterocephalus glaber),” Aging Cell, 9:626-35, 2010.] Instead of starting to proliferate in an uncontrolled manner, transformed naked mole-rat cells immediately stop dividing, though they do not die.[7. K.N. Lewis et al., “Stress resistance in the naked mole-rat: the bare essentials,” Gerontology, in press, doi:10.1159/000335966, 2012.] Similarly, naked mole-rat cells treated with a toxin or simply housed under suboptimal conditions immediately stop dividing until conditions improve.
Unlike mice, which very commonly develop tumors, naked mole-rats have never been found to naturally have cancer.
This has led some scientists to suggest that naked mole-rat cells are claustrophobic in culture and stop dividing as soon as they touch other cells, and that this contact inhibition is a mechanism of cancer resistance. However, several different labs have now shown that naked mole-rat cells grow to even higher densities than do mouse cells under optimal conditions, and do not avoid cellular contact under these circumstances. Rather, it has become increasingly clear that naked mole-rat tissues are better able to recognize abnormal cells, neutralize their tumorigenic properties, and repair their DNA. Should that fail, the cells are ushered into programmed cell death pathways.
The recently sequenced genome of the naked mole-rat has afforded a number of novel insights into why naked mole-rats appear to be impervious to cancers.[8. E.B. Kim et al., “Genome sequencing reveals insights into physiology and longevity of the naked mole rat,” Nature, 479:223-27, 2011.] Many of the genes involved in the regulation of cell proliferation are positively selected for or have unique sequences that appear to result in the naked mole-rat’s unusual health. Similarly, many gene families in the mole-rat genome are involved in DNA repair and detoxification processes, and the expression of these genes remains unchanged as the animals age. Given that cancer is one of the largest contributors to mortality in elderly humans, sustained genomic maintenance and simultaneous invulnerability to cancer may contribute substantially to the exceptional longevity of naked mole-rats.
Naked mole-rats also have in place several mechanisms to ensure protein quality control and homeostasis. Their proteins appear to be very resistant to unfolding stressors such as high temperatures and urea, and the animals’ cells are particularly efficient at removing damaged proteins and organelles via the ubiquitin-proteasome system and autophagy. Indeed, naked mole-rat proteasomes are both more abundant and show greater efficiency in degrading stress-damaged proteins in liver tissue than do the proteasomes within liver tissues of laboratory mice.[9. K.A. Rodriguez et al., “Altered composition of liver proteasome assemblies contributes to enhanced proteasome activity in the exceptionally long-lived naked mole-rat,” PLoS ONE, 7:e35890, 2012.] Similarly, autophagy occurs at a twofold greater rate in naked mole-rat cells than those of the mouse. Collectively, these enhanced intracellular cleaning processes may contribute to the better maintenance of a high-quality proteome and help the naked mole-rat’s cells resist damage in the face of cellular toxins, such as heavy metals or direct DNA-damaging agents. Much higher concentrations of these toxins are needed to kill naked mole-rat cells than are needed to kill mouse cells subjected to the identical experimental treatment.
Although naked mole-rats are the size of a mouse, weighing only about 35–65 grams, in captivity these rodents live 9 times longer. With a recorded maximum lifespan of 32 years, they are the longest-lived rodents known.10 And remarkably, they appear able to maintain good health for most of their lives. At an age equivalent to a human age of 92 years, naked mole-rats show unchanged levels of activity and metabolic rate, as well as sustained muscle mass, fat mass, bone density, cardiac health, and neuron number. These clear indications of both attenuated and delayed physiological aging are also accompanied by the maintenance of protein quality and gene expression levels.
Some of the oldest naked mole-rats (>26 years; equivalent to humans >105 years old) do begin to show signs of muscle loss, osteoarthritis, and cardiac dysfunction, demonstrating that mole-rats do, eventually, age like other animals. Somehow they delay the onset of aging and compress the period of decline into a small fraction of their overall lifespan. These findings of sustained good health are surprising given that the naked mole-rat is an exception to many of the current theories of why we age. For example, the widely accepted oxidative stress theory of aging attributes the gradual decline in function to damage caused by the free radicals or reactive oxygen species formed as an inevitable by-product of oxygen respiration. In much the same way that oxygen causes metal to rust when exposed to the elements, cell membranes, proteins, and DNA are damaged by the gas, and this accumulating damage, so goes the theory, causes physiological systems to malfunction. Naked mole-rats in captivity, however, show very high levels of oxidative damage at an early age, yet cellular function is not impaired, and the animals are able to tolerate these high levels of oxidative damage for more than 20 years.
Another aging theory posits that the length of an organism’s telomeres, the repetitive DNA that caps the ends of chromosomes, is a biomarker of aging and will correlate with species’ life span. But compared to the much shorter-lived laboratory mouse, the naked mole-rat has relatively short telomeres—similar in length to those of humans, in fact. Alternatively, cellular levels of telomerase, a reverse transcriptase enzyme that extends telomeres, may correlate with species longevity. But while telomerase activity has been measured in mole-rat skin cells in culture, it is generally very low, and is limited to those tissues that are actively replicating, such as testes, spleen, and skin. Thus, telomere length or maintenance is unlikely to explain the exceptional longevity of the naked mole-rat.
Clearly, studies involving this bizarre-looking but fascinating animal have highlighted many key facets of their unusual biology that are directly relevant to biomedical research. Indeed, these studies have yielded critical information regarding how the brain works, and how animals respond to the lack of oxygen and of light, as well as how we might learn to slow down aging, prevent cancer, and mitigate inflammatory pain and the harmful effects that occur when oxygen delivery is impaired. It will be exciting to be a part of the continued research on these incredible creatures that is likely to reveal novel drug targets for a variety of human ailments.
Thomas Park is a professor of biological sciences and neuroscience at the University of Illinois at Chicago. Rochelle Buffenstein is a professor of physiology at the Barshop Institute for Longevity and Aging Studies and the University of Texas Health Science Center in San Antonio, Texas.