Friday, June 21, 2013

The Krogh Principle in Action: Naked Mole Rats and Cancer

A naked mole rat
We have a lot to learn about the world around us.  The number of organisms living on earth is vast, as is the potential for us to learn about ourselves by studying their biology and physiology.

Physiology is the study of the function of living organisms.  Comparative physiology is a branch of physiology that examines differences between various organisms to better understand human physiology and diseases.  The philosophy of comparative physiology is often credited to August Krogh, a Danish scientist and Nobel Prize winner (Physiology or Medicine category; 1920).  However, this philosophy doesn't just apply to physiology.  It applies to all aspects of biology.  In this blog post, we'll discuss how a group of researchers compared cells derived from humans, mice, and naked mole rats to come up with a novel pathway for targeting cancer.  Read more after the jump....    

In 1929, Krogh wrote an article for The American Journal of Physiology about the current state of physiology.  He talked about the use of animals to solve physiological problems:

Prof. August Krogh
For such a large number of problems there will be some animal of choice or a few such animals on which it can be most conveniently studied.  Many years ago when my teacher, Christian Bohr, was interested in the respiratory mechanism of the lung and devised the method of studying the exchange through each lung separately, he found that a certain kind of tortoise possessed a trachea dividing into the main bronchi high up in the neck, and we used to say as a laboratory joke that this animal had been created expressly for the purposes of respiration physiology.  I have no doubt that there is quite a number of animals which are similarly "created" for special physiological purposes, but I am afraid that most of them are unknown to the men for whom they were "created," and we must apply to the zoologists to find them and lay our hands on them. 

[Please note that Krogh's use of the term created in quotation marks was a metaphor and not a reference to creationism]

Krogh was saying that a lot of biological problems can be understood by finding the right animal in which to study them and apply the currently available technology.  This idea is referred to as the "Krogh Principle."  By using the right animal to perform studies that can't be performed on humans, we can learn a lot about human biology.  But, it is not just the similarities between humans and animals that are important.  It is also the differences that are important.  In the same article, Krogh wrote about the study of kidney function, which was much less  understood in 1929 than it is was today:   

We may find out, nay, we will find out before very long the essential mechanisms of mammalian kidney function, but the general problem of excretion can be solved only when excretory organs are studied wherever we find them and in all their essential modifications.  Such studies will be sure, moreover, to expand and deepen our insight into the problems of the human kidney and will prove of value also from the narrowest utilitarian point of view.  

Krogh knew that understanding both the similarities and differences between different organisms can not only help us understand how humans function but also give us insights into what happens when human function goes awry.  A few paragraphs later in the article, he wrote that "you will find in the lower animals mechanisms and adaptations of exquisite beauty and the most surprising character."  There's a lot to learn by studying these some of these "exquisite" adaptations.  

Hans Krebs, another famous physiologist, broadly summed up the Krogh Principle as "certain species lend themselves especially well to the study of certain problems."  The physiologist C. Ladd Prosser defined comparative physiology as "not so much a defined discipline as a viewpoint, a philosophy" from which "biological generalizations can be reached as by no other method."   

The Krogh principle came to mind today when I was reading about a paper just published in the journal Nature that we will discuss below.  A lot of people in our society believe that we don't need to study the physiology, cell biology, or biochemistry of other animals, and that all we need to know we can gain from growing human cell lines or other existing cell lines.  Unfortunately, this just isn't the case.  These people are misinformed and unaware of how scientific discoveries often come about.  There is a popular misconception that animal research has no relevance to humans or that animal research is just plain unnecessary.  The fact is that we need many kinds animal research to better understand biology in order to come up with better treatments for human disease.  Of course animal research needs to be done as humanely as possible and with great care and caution, but just like August Krogh write on 1929, animal research can teach us a lot of useful information about human biology and physiology.  

Naked mole rats in a tunnel
I'm going to tell you a story about a very interesting discovery that came from studying a unique little animal called the naked mole-rat.  Naked mole rats (species name Heterocephalus glibber) are unique little rodents about the size of a mouse that live underground.  While it is thought that the earliest fossils of this species date back approximately 26 million years, they were first described by scientists in 1842.  However, it was debated at the time whether these animals were a real separate species or just either the juvenile form of some larger rodent species or members of some other rodent species that had lost their fur due to some kind of disease.  Naked mole rats have brownish pink loosely wrinkled skin that is mostly hairless (hence the "naked" in their name) except for a few hairs around their face, feet, and tail that are used to sense vibration and touch.  These animals are found in Africa and live underground in extensive mazes of tunnels and room that they dig out. 

These are very complex animals.  Their tunnels have been reported to extends as much as a mile long.  They even dig specialized tunnels such as drainage and ventilation tunnels and "toilet chambers."  They have complex methods of vocal communication to signal alarm or respond to cries for food from pups.  They live mostly on plant roots, bulbs, and tubers found while digging tunnels.  Some of their behavior has even been described as "farming."  They will gnaw parts of larger plant roots and tubers off, patch the holes with soil, and let them regenerate and regrow to create more food later on.         

These animals were first brought into a laboratory in the 1960s by a scientist named Jennifer Jarvis, who observed that naked mole rats are highly social animals who cooperate to find food and dig tunnels.  As scientists began to study the naked mole rats in more detail, they found that these animals have remarkably slow rates of aging.  They can live for approximately 30 years in captivity, which is about nine times longer than a laboratory mouse.  It has also been suggested that naked mole rats spend over 85% of their lives in a state of "good health" with high levels of physical activity well into their 20s. 

Naked mole rats have used as an example to support an evolutionary theory about aging that suggest that, when animals live under conditions of low external danger and few external causes of mortality (ie, few predators), their physiology will evolve to be focused on longevity and extended maintenance of their tissue.  Naked mole rats face few predators within their warm, sealed underground burrows, and so their bodies have evolved with the focus of staying healthy during their long lives.  They have very few changes in metabolism, bone mineral density, or body composition as they age.  This is in contrast to animals who live in more dangerous conditions and are more likely to die earlier from predators or harsher climates, where it has been suggested that evolution will favor development of characteristics that allow for earlier reproduction during their shorter lifespans.          

A common mouse (species Mus musculus)
The long lifespans of naked mole rats has been very interesting to scientists.  For many rodents like mice and guinea pigs, tumors are a major cause of death in later life.  Some strains of laboratory mice have a cancer mortality rate as high as 90%.  This is in stark contrast to the naked mole rat.  A multi-year study examining several laboratories that bred or housed thousands of naked mole rats found no evidence that any of the animals ever got any tumors.  None.  Several groups of scientists got the idea that, by studying naked mole rats, they might gain some insights into ways to suppress cancers.  They might gain insights that could then be applied to humans. 

It turns out that naked mole rats have developed an extra defense mechanism against the generation of tumors.  Tumors are basically caused by an uncontrolled, improper growth of cells, usually the result of some genetic mutation that prevents cells from knowing when to stop dividing.  One method of stopping cell growth is called "contact inhibition."  This is the phenomenon by which cells stop growing when they come into contact with each other.  This can be viewed in lab when cells are grown in a culture dish.  If cells in a lab are sparsely seeded onto the culture dish such that they aren't covering the dish, they will usually grow until they form a layer that covers the dish.  This state of covering all the available growing surface is called confluence.  A non-cancerous cell will then usually stop dividing when it comes into contact with other cells.  However, cancer cells will often continue to divide and grow because they have lost their contact inhibition.  Contact inhibition is an important anticancer mechanism to prevent abnormal cell division and growth of tumors.  The loss of contact inhibition is an important step in the generation of a tumor (called "tumorigenesis").   

Naked Mole Rat cells grow more slowly and stop diving at a lower density
Scientists who studied fibroblast cells from naked mole rats initially observed that these cells grew very slowly in culture.  Fibroblasts are cells that synthesize extracellular components such as collagen and are very important in wound healing.  Fibroblasts are frequently studied because they are easy to isolate and grow.  In 2009, these researchers published a paper describing how  naked mole rat cells exhibit contact growth inhibition that occurs at very low cell densities, meaning that these cells stopp dividing when they are further away from each other than would normally be seen with mouse or human cells.   Furthermore, when they tried to turn the naked mole-rat fibroblasts into cancer-like cells by introducing a mutated form of the Ras protein associated with cancer as well as part of a virus that can cause cancer (SV40) into them (a process called "malignant transformation"), they found that the naked mole-rat cells were very resistant and for the most part remained like normal cells. 

Now, in 2013, a new paper authored by many of the same researchers has been published in Nature.  They found that the molecular basis of this early contact inhibition and cancer resistance is due to the naked mole rat fibroblasts secreting a molecule called hyaluronan that is over five times larger than the hyaluronan secreted by mouse or human cells.  Hyaluronan is large sugary molecule that is secreted by cells and can form large chains to make up the "extracellular matrix," the non-cellular part of tissues that provide structure and support for cell growth.  Connective tissues are largely made up of extracellular matrix, and hyaluronan is an important component of the extracellular matrix of cartilage and skin.  Hyaluronan plays an important role in tissue elasticity.  These researchers named the extra-large naked mole rat hyaluronan "high-molecular-mass hyaluronan." 

Part of this study was spurred on by the researchers' observation that, when culturing the naked mole-rat fibroblasts, the culture media (the nutrient and salt solution used to grow the cells) became very viscous (thick) after several days.  When they treated the media with hyaluronidase, an enzyme which digests hyaluronan, the viscosity went down.  These led researchers discovered that not only do naked mole-rat fibroblasts secrete a larger form of hyaluronan than do human and mouse fibroblasts, but they also make a decreased level of hyaluronidase.  This results in a greater accumulation of hyaluronan in their tissues and a higher skin elasticity.        

Cells have receptors for hyaluronan, and when they come into contact with hyaluronan in the extracellular matrix, these receptors are activated and initiate signals that can control cell growth and contribute to contact inhibition.  Not only do naked mole rat fibroblasts make extra-large hyaluronan that accumulates to higher levels, but their receptors for hyaluronan are also more sensitive to hyarluronan.  These researchers found that if they took the naked mole-rat fibroblasts and inhibited their ability to make high levels of this "extra-large" hyaluronan, the cells then became more susceptible to malignant transformation--they more easily were turned into cancer-like cells--just like human and mouse cells. 

The authors of this study concluded that, as the naked mole rats evolved to accumulate a higher concentration of "extra-large" hyaluronan, they ended up with dual benefits: increase skin elasticity for their underground life and the anticancer properties that result in their increased longevity. 

While we don't know exactly if or how any of these findings can be translated to clinical anti-cancer drugs for use in patients, this research opens up a new avenue of investigation to pursue in the war against cancer.  It is very possible that this special form of hyaluronan could turn out to be a useful anti-tumor drug or give us insights into how to restore contact inhibition and stop the growth of cancer cells. 

Would they have found any of this out without studying the naked mole rat?  Absolutely not.  Once again, the wisdom of the Krogh principle shines through:  Studying similarities and differences between ourselves and other animals can teach us a lot about the function of our own bodies in health (physiology) as well as in disease (pathophysiology).  The naked mole rat may be the only animal in existence that utilizes this unique type of hyaluronan to prevent tumor growth, and had researchers not studied the naked mole rats, we would never even know this exists.  This is an example of one of the "adaptations of exquisite beauty and the most surprising character" in a "lower animal" that Krogh wrote about.  It is also a prime example of why we need animal research to discover new potential treatments for human disease and hopefully improve human lives.

We may hear more of the naked mole rat in the future, as another recent study even suggests it may teach us something about the pathogenesis of Alzheimer's disease, but that's a topic for another post...           
Text © 2013 TheMadScienceBlog; Images are public domain.  

Sources and Further Reading
  • J. Azpurua and A. Seluanov.  "Long-lived cancer-resistant rodents as new model species for cancer research."  Frontiers in Genetics.  2013.  3:319.  Available here.
  • R. Buffenstein, T. Park, M. Hanes, and J.E. Artwohl.  "Naked Mole Rat."  Chapter 45 in The Laboratory Rabbit, Guinea Pig, Hamster, and Other Rodents.  2012.  Elsevier, Inc.  DOI: 10.1016/B978-0-12-380920-9.00045-6     
  • Y.H. Edrey, M. Hanes, M. Pinto, J. Mele, and R. Buffenstein. "Successful aging and sustained good health in the naked mole rat: a long-lived mammalian model for biogerontology and biomedical research." ILAR Journal.  2011. 52:41–53.  Available here.
  • Y.H. Edrey, D.X. Medina, M. Gaczynska, P.A. Osmulski, S. Oddo, A. Caccamo, and R. Buffenstein.  Amyloid beta and teh longest-lived rodent: the naked mole rat as a model for natural protection from Alzheimer's disease."  Neurobiology of Aging.  2013.  In Press. 
  • H.A. Krebs.  "The August Krogh Principle: For Many Problems There Is an Animal on Which It Can Be Most Conveniently Studied."  Journal of Experimental Zoology.  1975.  194:221-226.
  • A. Krogh.  "The Progress of Physiology."  The American Journal of Physiology.  1929.  90:243-251.
  • R. Lipman,A. Galecki,D.T. Burke, and R.A. Miller. "Genetic loci that influence cause of death in a heterogeneous mouse stock." J Gerontol A Biol Sci Med Sci. 2004.  59:977–983. 
  • C.L. Prosser.  "Prospects for Comparative Physiology and Biochemistry."  Journal of Experimental Zoology.  1975.  194:345-348. 
  • A.  Seluanov, C. Hine, M. Bozzella, A. Hall, T.H. Sasahara, A.A. Ribeiro, K.C. Catania, D.C. Presgraves, V. Gorbunova.  "Distinct tumor suppressor mechanisms evolve in rodent species that differ in size and lifespan." Aging Cell.  2008.  7:813– 823.  Available here.
  • A. Seluanov, C. Hine, J. Azpurua, M. Feigenson, M. Bozzella, Z. Mao, K.C. Catania, and V. Gorbunova.  "Hypersensitivity to contact inhibition provides a clue to cancer resistance of the naked mole rat."  Proceedings of the National Academy of Sciences of the USA. 2009. 106:19352–19357.  Available here.
  • X. Tian, J. Azpurua, C. Hine, A. Vaidya, M. Myakishev-Rempel, J. Ablaeva, Z. Mao, E. Nevo, V. Gorbunova, and A. Seluanov.  "High-molecular-mass hyaluronan mediates the cancer resistance of the naked mole rat."  Nature.  2013.  In Press.  doi:10.1038/nature12234 


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