Evolution’s Surprise Bag: The African Naked Mole Rat
This beauty wouldn’t win any pageants, but it is a fascinating animal. The African naked mole rat (Heterocephalus glaber), whose hairless, tubular, wrinkled body makes it appear a bit like a tiny walrus lives in underground burrows in very dry areas of East Africa (Kenya, Ethiopia and Somalia).
Naked Mole Rat in a Zoo
Neither Mole nor Rat
Eduard Rüppell, a German naturalist of the 19th century, who first documented a naked mole rat, assumed from its unprepossessing appearance that it was a diseased or mutated individual of another species.
Taxonomically speaking...the name is misleading it is neither mole nor rat: It lives in underground burrows the way moles do. Yes, it has a skinny, rat-like tail. Yet the naked mole-rat is more closely related to porcupines, chinchillas, and guinea pigs than it is to moles or rats.
And yes it has hair. It's hard to see, but naked mole-rats do have about 100 fine hairs on their body that act like whiskers to help them feel what's around them. Hairs between their toes help sweep soil behind them while tunneling. And since they spend their lives underground, they don't need hair for sun protection.
A Skin adapted for Living Underground
While the temperature of the desert above changes from scorching during the daytime, to very cold at night, their burrows remain at an almost constant temperature. This is important because unlike most other mammals, naked mole rats can't maintain steady body temperature - they lack sweat glands and are essentially cold-blooded. If it does get cold at night, the little mammals just huddle together in a mole-rat pile and use each other's body heat to keep warm.
When the naked mole rat slithers through narrow openings the loose, wrinkled skin, much like the loose-fitting clothing a spelunker wears, helps the mole rat to avoid abrasions. In addition the skin cannot detect pain from an acid burn or capsaicin because naked mole rats lack the neurotransmitter substance P that in every other mammal helps relay pain signals from the skin to the central nervous system [1].The animal reacts normally to the mechanical pain caused by pinching and prodding,
Naked Mole Rats live in Eusocial Communities like Ants
Naked mole rats are rodents, but they live in eusocial communities like ants. 80 to 300 naked mole rats live together in colonies led by one dominant female mole rat—the queen. As in ants, the queen is the only female to breed and bear young.
Research is still being carried out to discover how the queen stops other females in the colony from breeding. Scientists wondered if it was done by pheromones, but it became clear she achieves precedence just by her behavior rather than any pheromones or whatever.
However it's done, the effect is only temporary. When the breeding female dies, other females in the colony begin to develop eggs within 7-10 days of her death. The older females are the first to develop eggs and often fight to the death for the right to become the breeding female. The winner grows in size, mates with the breeding males and becomes the new queen of the colony.
Only two to three males are allowed to mate with the queen. The other naked mole-rats in the colony serve as soldiers or workers. The mole-rats assigned as workers care for the queen's pups, dig the tunnels, and scout for food. If a naked mole rat finds food it runs back to its mates, leaving a chemical trail along the route.
If a snake or other predator comes into a tunnel, the workers let out an alarm call to alert the soldiers. Like a little army unit, the soldiers run off to defend the colony with their large, sharp teeth. Several of the soldiers may pile on top of each other to block the way, and all the predator sees is many sets of gnashing teeth!
Naked mole-rats and ants are, perhaps, the best living example of what biologists call convergent evolution: the development of nearly identical characteristics in completely unrelated species.
The Underground Naked Mole Rat Housing Estate
A typical colony tunnel network might run to 4 km in an area the size of a football pitch and the mole rats, which can run as fast backwards as they can forwards, will shift three or four tonnes of earth in the few weeks after it rains. There are specialized subterranean chambers specifically dedicated to rearing offspring, storing food, and eliminating bodily waste — there are even major "highway systems" — complete with on-ramps and off-ramps — that allow for more than one animal to travel quickly over vast underground distances.
The air in underground colonies of naked mole rats is disgusting and limited, high in carbon dioxide and low in oxygen. If you had to breathe it, you would not only be grossed out, but you'd get brain damage. Yet these animals have adapted to survive in low-oxygen environments. In fact, they can go more than a half hour under extreme oxygen deprivation — a condition known as hypoxia, without damage to brain cells [2].The animals never need to drink, because they get all the fluid they need from water-rich food and the moist burrow air.
A Research Model for Evolutionary Medicine
Naked mole-rats live longer than any other rodent, with a life span of nearly 30 years in zoos a lifespan eight times that of similarly-sized mice. Closer study revealed that not only did naked mole rats live a long time, they also resisted almost all typical signs of ageing: They show no age-related increase in mortality, and have a high fecundity until death [3]. They demonstrate a healthy longevity that all of us would like to emulate. In addition they have never once been observed to develop cancer, even in the presence of carcinogens.
Scientists hope that studying a species so long-lived (particularly given its small body size) and with such an astonishing resistance to tumors, will help elucidate mechanisms and genes conferring longevity and cancer resistance in mammals that may have human applications. A project entitled “Functional genomics of the naked mole-rat” has the goal to develop genomic resources for the naked mole-rat, clone and characterize selected naked mole-rat genes [4] and ultimately create transgenic mice with naked mole-rat genes. Such knock-in mice will be exceptional models to study adaptations of naked mole-rats related to aging and age-related diseases like cancer.
The Mighty Quest for Longevity and Youthfulness
The widely accepted oxidative stress theory of aging postulates that aging results from accumulation of oxidative damage. Surprisingly, naked mole-rats, when compared with mice exhibit higher levels of lipid peroxidation, protein carbonylation, and DNA oxidative damage even at a young age [5]. In naked mole-rats neither lower levels of reactive oxygen species are present [5], nor do anti-oxidant levels appear significantly raised [6]. Despite this, phospholipids are less damaged by oxidation in naked mole-rats compared to shorter-lived rodents [7]. In comprehensive testing and analysis of oxidation states of protein cysteines, the researchers found that, compared with mice, mole rats had higher levels of total cysteine and no age-related changes in cysteine oxidation during more than 20 years [8]. The mole rats also showed unusual resistance to protein unfolding and lower levels of protein degradation during aging [8]. In naked mole rats, proteins sustain oxidative damage early on yet remain stable throughout the rats’ long lives, the scientists found. In contrast, proteins in short-lived mice show increasing levels of oxidative damage as they get older. The results suggest a new biochemical mechanism underlying longevity: the ability of oxidized proteins to maintain their structural stability and integrity.
In recent years, Vera Gorbunova and her husband Andrei Seluanov at the University of Rochester, New York, USA tried to figure out how exactly the naked mole rat manages to live so long. As revealed in research her team published in PNAS [9], their team thinks they’ve found at least part of the answer: naked mole rats have strange ribosomes.
The ribosomes in almost every multicellular organism on the planet is made up of two large pieces of RNA. But last year, one of the Rochester lab’s students was isolating RNA from cells taken from the naked mole rats when he noticed something unusual. When he separated the RNA pieces, instead of seeing two distinct pieces of ribosomal RNA, he saw three. It turned out that, compared to mouse ribosomes, these three-part structures made between four and forty times fewer errors during the translation process. At this point, its unclear how exactly that might lead to longer lifespans, but the researchers believe it plays a key role.
The Secrets of Cancer Resistance
With her team, Gorbunova identified the fact that cells of naked mole rats display a very high degree of proximity inhibition – they don't like to grow close together. This inhibition was proved to be the result of a complex sugar called hyaluronan (HMW-HA) [10], which is present in all mammals, filling the gaps between cells, but which naked mole rats produce in abundance. The molecular structure of their HMW-HA is many times larger, and they are slower at recycling it, meaning it builds up in a unique way, giving the naked mole rat the ability, among other things, to stretch their skin extremely long. The same molecules that endow naked mole rats with springy, wrinkled skin also seem to prevent the homely rodents from contracting cancer. The researchers suspected that the long HA molecules formed a tight cage around cells, preventing tumor cells from replicating unchecked and essentially nipping 'pre-cancers' in the bud.
In addition researchers think a tumor suppressor gene that codes for a protein named p16Ink4a also plays a role in tumor prevention [11]. The p16 protein, like p27 in humans, works by keeping groups of cells (like pre-cancerous growths) in check, and prevents them from proliferating. The difference is that while humans rely predominantly on p27, naked mole rats rely on both. P16 is as an additional checkpoint in the body's defense against cancer,
References
Photo Credits
Author: Roman Klementschitz, Wien
Date: 31 December 2003
License: This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.