Science in Writing: Animal Physiology

 

In Kenyon biology, students learn not only how to perform scientific inquiry but how to communicate their science clearly and effectively. Dr. Christopher Gillen has a passion for understanding how animals work, and his research specializes in salt and water balance physiology, most recently examining salt absorption and secretion in the Aedes aegypti mosquito. His course Biology 243: Animal Physiology is one of the most popular in the department, and students compare complex physiological processes across different organisms under Dr. Gillen’s enthusiastic instruction. In addition to his passion for understanding how animals work, Dr. Gillen is passionate about making scientific research understandable and enaging for all audiences and he is the faculty director of the Kenyon Institute in Biomedical and Scientific Writing. This semester, he launched Kenyon’s first science writing seminar course with Professor Sergei Lobanov-Rostovsky of the English department, where students read and discuss a wide variety of literature with a scientific focus, write voraciously on their own scientific fascinations, and experiment with the many techniques and nuances of the science writing genre.

Dr. Gillen stresses the importance of communicating science in all of his classes, particularly how to write about science for a variety of audiences:

“Pitching complicated research to a general audience is hard. Students must understand the science deeply and frame it with compelling writing and storytelling. And the skills they learn writing for a general audience transform them into better scientific writers.”

In Animal Physiology, students were asked to complete a News and Views assignment where they write two essays on the same scientific research article: one essay that makes an argument about the research to an audience of scientists and another essay that explains the research to a broader audience of non-scientists. Here is one such article for a general audience, written by junior neuroscience major John Wilhelm:

Glowing Intestines and Digestive Sledgehammers: the Cat’s Digestive Toolbox

By John Wilhelm

As it turns out, science backs what many of us were saying all along—dogs are definitely sweeter than cats. We just didn’t know it had to do with their rates of glucose uptake.

While dogs are notoriously the dinner table freeloaders—sniffing for crumbs, begging for any scraps they can get—cats tend to be indifferent at mealtime. If your cat is a lunchtime beggar, chances are it’s also a chooser—cats have little interest in peanut butter, chocolate, or sugar; he’s probably after some meat or fish. Is it natural feline aloofness that allows them to resist sugary food? A recent study1 by researchers at the University of Pennsylvania School of veterinary medicine reveals it might have less to do with cats’ attitude and more to do with their intestines.

Compared to dogs, cats have a difficult time processing glucose. Like many mammals, dogs are omnivores—they tend to be less picky eaters; many will eat anything they can get their paws on. Cats, on the other hand, are obligate carnivores—their bodies are designed to process meat better than anything else. The researchers examined the biological basis of this by looking at the intestinal makeup of domestic dogs and cats. Though the dietary needs of Fido and Felix might not seem such a grandiose topic, and “cat diabetes” might not strike you with appropriate gravity, there’s a lot involved in helping our furry companions lead long and healthy lives. On top of that, the digestive systems of mammals are very comparable—most of us work with same “toolbox” of digestive enzymes. A comparative study like this helps us better understand mammalian digestion as a whole.

When dogs and cats eat carbs, small enzymes along the walls of the intestine facilitate their breakdown and absorption. The enzyme that actually brings glucose into the body is the star of the show—sodium-glucose cotransporter 1 (SGLT1). The SGLT1 hangs out on the intestinal wall, looking for glucose molecules to bind. When the enzyme snags a molecule of glucose, it inverts itself and essentially “flips” the glucose (and an additional sodium molecule) into the intestinal cells. The action of this enzyme determines how quickly glucose can enter the body, which means it can determine how much energy an organism has access to.

Of course, most of the food that dogs and cats eat doesn’t come readily in glucose form. Animals eat more complex chains of carbohydrates, called polysaccharides. These chains are broken down mostly in the mouth and stomach, but the final slice occurs in the intestine, where disaccharides are split into two monosaccharides. The enzymes that make that cut are called disaccharidases. In the toolbox of digestive enzymes, disaccharidases work closely with the sodium/glucose cotransporter in order to chop up carbohydrates and flip them into the body.

To quantify these enzymes, the University of Pennsylvania researchers used a technique called immunohistochemistry—they treated frozen intestine samples with special antibodies that stick to the digestive enzymes. The researchers can then measure the antibodies in different ways to get a sense for the amount of digestive enzymes. For example, some of the antibodies produce light—so the researchers can, literally, measure how much the intestines glow. Of course, you might ask yourself why researchers would go around sticking antibodies to enzymes rather than measuring the enzymes themselves. The answer is fairly simple—the antibodies are designed to be easy to detect and measure, and most of our internal enzymes are not.

Ultimately, the researchers found that—though their toolkits are similar—where cats are working with mallets, dogs have sledgehammers. Cats express 200% less SGLT1 than dogs do, which means they uptake carbohydrates a great deal slower. Additionally, they express between 150-400% fewer disaccharidases, which means they’re also way less effective at breaking down those carbs. This limits cats’ diets significantly—that means little to no fruit, veggies, and grains.

This research builds upon earlier work at the University of Pennsylvania3 on two other enzymes—T1R2 and T1R3. These enzymes work together to form the “mammalian sweet receptor, ” which allows mammals to taste (and generally approve of) sweet foods. Though cats’ taste receptors are fairly comparable to dogs, there’s one major exception—they don’t express T1R2, which means they can’t taste sweet food at all. Of course, this is just molecular confirmation that cats live in a very different universe than their owners.

Thankfully, cats need not live the Sisyphean life of enjoying a delicious carbohydrate but being awful at digesting it. It might even seem like cats’ indifference to carbs is advantageous, since obesity is such a ubiquitous problem for humans. While they aren’t as enchanted by sugar as us, their bodies also haven’t adapted to process it like most mammals. This predisposes cats to both hyperglycemia and diabetes5—indeed, the ‘fat cat’ stereotype does have a biological basis!

For the researchers, this presents an interesting finding—it gives us some insight into the different ways that mammals have developed their digestive toolkits. For cats, this means they ought to eat mostly protein, though some mainstream dry cat food brands still include more carbohydrates in their food than is healthy. For dogs, this means that perhaps your puppy has a little bit more of a reason to go around trying to eat anything that fits in his mouth. He’s got the enzymes to do it. For pet owners, this means that a vegan Felix is a sick Felix, and that you shouldn’t feel so bad next time you’re shunned by a feline friend—it seems they don’t even know what it means to be sweet, and their intestines can hardly handle it.

Read the original research articles here:

Sodium/glucose cotransporter-1, sweet receptor, and disaccharidase expression in the intestine of the domestic dog and cat: two species of different dietary habit (2011) – D. J. Batchelor, M. Al-Rammahi, A. W. Moran, J. G. Brand, X. Li, M. Haskins, A. J. German, S. P. Shirazi-Beechey

http://ajpregu.physiology.org/content/300/1/R67

Dietary regulation of intestinal brush-border sugar and amino acid transport in carnivores (1991)- R. K. Buddington, J. W. Chen, J. M. Diamond

http://ajpregu.physiology.org/content/261/4/R793

Pseudogenization of a Sweet-Receptor Gene Accounts for Cats’ Indifference toward Sugar (2005)– X. Li, W. Li, H.Wang, J. Cao, et.al.

http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.0010003

 

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