by Daniel Engber, New York Times,
posted online 5/17/17, Will appear in print in Sunday May 21st New York Times
Magazine Section.
As Amit Choudhary opened the
package of snake blood, the first thing he noticed was its color. It looked
like yogurt. The blood plasma, harvested from a Burmese python shortly after
feeding, was so clogged with fatty acids that it was not clear but milky white.
An oily mess like that should be toxic, Choudhary thought. Indeed, when he
smeared the same amount of fatty acids on a plate of human pancreatic cells,
the kind that supply the body with insulin, they self-destructed from the
stress. Yet he knew the snake could somehow thrive, even as its plasma turned
to yogurt after every single meal.
It was the late fall of 2012, and
Choudhary, then a postdoctoral fellow at Harvard University, had become
interested in the Burmese python on account of its extraordinary physiology.
The animal is essentially a slithering digestive tract: In the wild, it often
spends a month or two in silent ambush; then, when the moment is right, it
wraps its coils around a monkey, a pig, an antelope and swallows its prey head
first. A single meal for a full-grown python may contain more than 50,000
calories, a tidal wave of nutrients and fatty acids that could be deadly to
another species. But the python has adapted to the overload. For the week or so
that follows feeding, its body turns into an engine of digestion: Its intestine
thickens; its liver and kidneys nearly double in mass; its insulin level shoots
up; its temperature increases by six degrees Fahrenheit; its pulse triples; and
its metabolism jumps. Once all the food has been absorbed, the python’s organs
shrink back to their quiescent state.
In his lab, Choudhary wondered if
there might be something in a python’s yogurt blood that helps it undergo this
protective transformation. He dabbed a few drops on a rodent’s pancreatic cells
to see how they would respond. “The results were mind-blowing,” he recalled on
a recent April afternoon in Boston, where he now does research at Harvard and
M.I.T.’s Broad Institute. Choudhary used an antibody kit to measure how much
insulin the rodent cells produced. The assay works by color: Insulin turns a
sample very slightly yellow, a change researchers detect using a specialized
instrument. But after Choudhary treated the cells with snake blood, they
started spurting so much insulin that he could see the sample changing color.
The python’s fatty plasma didn’t just insulate the pancreas from fatty acids;
it appeared to juice it up.
Growing up in a farming area
several hours from Kolkata, India, Choudhary had always been afraid of snakes.
“I was told not to mess with those creatures,” he says. Now he realized they
might hold a cure for diabetes. The pancreatic cells he used in the lab were
beta cells, and diabetes is a beta-cell disease. In the early-onset (Type 1)
form, the body’s immune system turns against these cells and nearly wipes them
out. Without a source of insulin, a patient can’t control the sugar in the
bloodstream. In Type 2 diabetes, the version of the ailment linked to Western
diets and obesity, it goes the other way: The patient loses sensitivity to the
insulin being produced, and the beta cells become overcharged to compensate,
until eventually they weaken or die. What if Choudhary could find a chemical in
snake blood that worked to bolster the beta cells and reverse the condition?
Choudhary knew the stakes as well
as anybody. In India, they call diabetes “having sugar,” and his family seemed
at special risk: Nearly every one of his male relatives, and some females, too,
had sugar, and many died from its complications. Even he and his younger
brothers, all of whom now live in the United States, may be doomed to the
condition in their later middle age. So after completing his dissertation work
in quantum chemistry and biophysics, Choudhary started looking into diabetes.
Until that point, he found, much
of the research into therapies for beta cells had been done on laboratory
rodents. After years of work on rats and mice, scientists had come up with lots
of drugs that energized a rodent’s pancreas, but very few of these showed any
promise when it came to treating humans. We don’t yet know why, but one reason
may be that the pancreas of a mouse or a rat is a bit peculiar. For one thing,
its beta cells are livelier than ours and more likely to divide. They are also
grouped together in one place in the pancreas, while ours are a bit more
scattered. And rodents have a pair of genes encoding insulin, while we have
only one.
In spite of these discrepancies,
rats and mice remain the most common research animals for the study of
diabetes, as they are for all of biomedicine. In part because of their
proximity to humans on the evolutionary tree, in part because of their small
size and in part because of our facility at playing with their DNA, Mus musculus and Rattus norvegicus have become the default choice for biomedical
experiments. But if sickly diabetic mice and rats were useful models of the
disease, Choudhary thought, might there be even greater value in a species that
evolved to be the opposite — a model of good health, resistant to whatever
makes a rodent or a person fall apart?
Choudhary had the snake blood and
a plate of beta cells on steroids. But before he allowed himself to dream of
wonder cures, he tried the same experiment again — putting milky drops into a
petri dish — this time using plasma from a different snake. Once again, the
beta cells were sent into a riot. If Choudhary were to start a brand-new
research program using laboratory pythons, he would need to be absolutely
certain that he hadn’t messed things up. “When the results are too good,” he
said, “you always worry that something must have gone wrong.” He ran the
experiment four more times. It always worked.
The
idea of using pythons to understand metabolism got its start 100 years ago, on
the very same street as Choudhary’s Harvard office. Opposite the medical school
on what is now called Blackfan Circle, a physiologist and professional magician
named Francis Gano Benedict embarked on a landmark 17-year study of absorption
in the snake.
Benedict
came to Boston on behalf of the Carnegie Institution to found a laboratory for
the study of nutrition. He brought with him a state-of-the-art device that he
helped develop while at Wesleyan, known as the respiration calorimeter. An airtight
chamber lined with zinc and copper, it could be used to calculate the energy in
food — i.e. its “calories” — and study how exercise or eating might affect a
person’s metabolic rate.
He
soon discovered that his measurements of energetics could be applied to nearly
any animal — he once built a mammoth version of the chamber for an elephant —
and used to answer endless questions on nutrition, important or mundane. At one
point, he calculated the energetic cost of thinking: “The cloistered scholar at
his books may be surprised to learn that the extra calories needed for one hour
of intense mental effort,” he announced in 1930, “would be completely met by the
eating of one oyster cracker or half of a salted peanut.”
As
part of his grand project on behalf of Carnegie, Benedict set out to construct
a bestiary of metabolic rates. In 1915, he traveled to New York City to install
a respiration chamber in the reptile house at the Bronx Zoo. He hoped to use
the snakes — including the zoo’s 17-foot python — to understand how and why an
animal’s body temperature might fluctuate. Eventually, he predicted, this work
could help explain the mysteries of human fever.
By
the following spring, Benedict had begun a study of the snakes’ response to
eating. It intrigued him that pythons and other constricting snakes, in
particular, could ingest an “inconceivably large mass” at one time. He had them
gobble rabbits whole, or hunks of beef fat wrapped in the pelts of guinea pigs,
and tested them in his giant-snake-size chamber. The resulting bumps in heat —
and fivefold increase in carbon-dioxide production — left him dazzled. “With no
other animals in the world, certainly among the vertebrates, can one expect
such economy in the taking of food,” he wrote in his monograph from 1932, “The
Physiology of Large Reptiles.” A photograph of Benedict, published that same
year, shows him in a lab coat and bow tie, proudly holding an eight-foot python
by its neck.
For
Benedict, the python’s Olympian feats of digestion made it worthy of study, but
he was nearly as impressed by its placid disposition — a highly valued trait in
a laboratory animal. Special credit went to the python he examined at the
National Zoo in Washington: “In the entire series of experiments with this
animal she gave but one sign of agitation,” he wrote, “which was a heavy, deep
breath, approximating a hiss, in the middle of an afternoon session.”
Yet despite Benedict’s efforts to
proselytize, the python barely made its way into the labs of other
physiologists. In fact, the details of its digestive tract and their
implications for our understanding of metabolism were more or less ignored for
60 years, until a young biologist named Stephen Secor started tracking
rattlesnakes in the Mojave Desert.
Secor, who grew up on a horse
farm, planned to be a veterinarian, but a love of snakes derailed him. At
first, he tried to study their behavior in the lab (the mating habits of the
speckled king snake), but he found the research boring. So for his Ph.D. at the
University of California, Los Angeles, Secor headed out into the field to track
a pair of wild species — the coachwhip and the sidewinder rattlesnake — and
learn how they managed their metabolism in a natural setting. As eaters, these
snakes were polar opposites. Secor noticed that the coachwhip, whose form is
long and slender, was always on the move, chasing little lizards through the
Kelso Dunes. A sidewinder waits for its dinner: It sits in ambush until a
kangaroo rat happens to come hopping by, and then it has a banquet.
When Secor presented his
observations at a 1991 scientific meeting at White Mountain Research Center, a
few hours north of Los Angeles, Jared Diamond raised his hand. Diamond, now better
known for his work as a geographer and historian (he is the author of “Guns,
Germs and Steel”), was at the time a physiologist in the mold of Francis Gano
Benedict. Since the early 1980s, he had been working on a grand survey of
animal digestion. His lab had looked at rats and mice, cats and minks, frogs
and fish. He had even studied how a hummingbird handles so much nectar in its
small intestine.
Diamond thought of the intestine
as something like a muscle, lifting substances into the bloodstream. If you had
to do more lifting, he reasoned, you would need to have a stronger muscle. So
Secor’s talk about the sidewinders, and their feast-or-famine diet, made him
wonder if the animals’ intestines were endowed with superhero strength. As it
happens, Secor had tried to measure the rattlesnakes’ metabolism, but the
numbers he got were so high that they seemed impossible. A mammal’s metabolic
rate typically rises by one-quarter or one-half after feeding. The largest such
increase anyone had ever measured in a reptile was a fourfold or fivefold
change. But Secor’s data showed that sidewinders could increase their
metabolism by almost eightfold — a bigger jump than anyone had seen in any
species. A prominent scientist at U.C.L.A., who would later serve as Secor’s
mentor, told him that he must be doing something wrong. “Leave the physiology
to physiologists,” the professor said.
But when Diamond looked at
Secor’s numbers, he believed them. “This is the most exciting data I’ve seen in
five years,” he announced. As soon as Secor finished his dissertation, he moved
his work to Diamond’s lab. It soon became apparent that, as research animals,
Secor’s three dozen wild rattlesnakes were not ideal. “Every now and again,
he’d get bitten, and then he’d get a little dizzy,” Diamond remembers. Secor
says the venom didn’t bother him much, but U.C.L.A.’s administration was not
keen on housing a colony of sidewinders. So Secor and Diamond set out to find a
safer snake with an athlete’s intestine. Secor tested a Noah’s ark of options —
a pair of water snakes, a pair of corn snakes, a pair of black racers, a pair
of Burmese pythons — before he found his champion. The python’s gut looked even
more impressive than the rattlesnake’s. After feeding, the intestine blew up
like a bodybuilder’s, doubling in mass.
And if the python’s meal was very
large, its metabolic rate would increase not just eightfold, like the
rattlesnake’s, but by a factor of 44. The only comparable metabolic increase
Diamond knew of had been identified in a galloping racehorse. The most
impressive short-term increase he had seen in humans — measured in water-polo
players and cyclists in the Tour de France — was no more than five or six times
their resting rate. “These pythons made the Tour de France cyclists and water-polo
players look like wimps,” he says. What’s more, the python could sustain that
level of metabolism for several days.
Secor kept a pair of 12-footers
in his home, Linus and Bob; he knew pythons were easy to acquire and maintain.
The pet trade in the Burmese python had been booming since the 1980s. So he
reached out to a guy he knew in Oklahoma, who happened to be the nation’s
biggest python breeder, and put in an order for 100 snakes. Over the next few
years, Secor worked on tracing how a Burmese python’s body transforms itself
after feeding. Nearly every facet of digestion was exaggerated, he found, from
the way the python fills its stomach with hydrochloric acid to the way its
organs thicken and expand. For him and Diamond, the scale of these effects had
a scientific value of its own. It made the process of digestion easier to
study, because every nuance could be seen in high relief.
But as Secor finished this
initial round of work, Diamond was preparing to move on. “The future of the
world does not depend on intestines and gallbladders,” he had decided. In the
fall of 1998, he and Secor summarized their research on pythons in the journal
Nature. Their article was half-prospectus and half-plea — a call to fellow
physiologists to make the Burmese python a standard laboratory animal. The
snakes were practical, the article said, on account of their infrequent feeding
and inexpensive upkeep, their convenient “linear anatomy,” their easygoing
nature and their relative lack of moral standing. (Snakes “do not arouse the
controversy associated with medical research on similarly sized mammals,” Secor
and Diamond noted.) But the Burmese python’s greatest value — its raison d’être
as a laboratory species — derives from its special way of eating.
“The history of biology
illustrates the importance of selecting exceptionally suitable species as
models,” Secor and Diamond wrote. The fruit fly, for example, with its rapid
reproduction, helped scientists understand genetics; the squid, with its
peculiarly gigantic axon fibers, allowed scientists to examine the workings of
a single nerve cell. Could pythons, with their astounding feats of regulation,
become the next great model animals?
Secor
now runs his own lab, bustling with reptiles, at the University of Alabama, in
Tuscaloosa. Since 1998, he has published 30 papers on the python. His lab has
studied, among other things, how the python’s microbiome changes in response to
eating; how its metabolism responds to meat depending on whether it has been
ground up like hamburger or cooked in a microwave oven; how it regulates its
blood flow after feeding; and how it keeps a rising tide of gastric acid from
lapping into its throat.
For
most of his career, Secor’s work has been unadulterated by the need for
real-world applications. People rarely bothered him with plans to plumb the
python as a source of pharmaceuticals or as a means to better human health. “I don’t
care about the medical aspects,” he says he might have told them at the time.
“That’s not what I do. I just love these animals because I’m interested in
their biology.”
Then,
all at once, the field began to change. First a group based at the University of
California, Irvine, led by a comparative physiologist named James Hicks,
published data showing that the python’s heart can grow in size by 40 percent as
the animal digests. Not long after, Leslie Leinwand, who studies cardiac
disease at the University of Colorado, became interested. Leinwand knew that in
humans, an enlarged heart can be a sign of good health — athletes get bigger,
stronger hearts from working out — but it can also signal that the organ is
diseased. With help from Secor, she and her postdoctoral researcher, Cecilia
Riquelme, found that after feeding, a python’s heart swells the way an
athlete’s does, with no ill effects. Then they tried pouring snake blood on a
dish of rat heart cells and found it bulked them up, like liquid exercise. A subset
of the fatty acids in the python’s blood seemed to
function
as a cell-expanding cocktail, one that worked even in living mice.
In
early 2012, Leinwand presented her research at the Broad Institute, where
Choudhary had just begun to think about the python as a tool for treating
diabetes. The python’s heart might swell by 40 percent after feeding, he
thought excitedly, but its pancreas would double. Leinwand had found a signal in
a python’s blood that makes heart cells big and strong.
What
if he could do the same, or more, for pancreatic beta cells? Later, in 2013,
another researcher, Nicholas Stylopoulos of Boston Children’s Hospital, whose
office happens to be just down the street from Choudhary’s (and from the site
of Francis Gano Benedict’s), decided that the python might be a useful model
for his work on the science underpinning gastric-bypass surgery and its
benefits for diabetics.
Why
did it take so long for medical researchers to see the value in the python? For
one thing, the conditions the python might help to treat have become more
prevalent. In 1998, when Secor and Diamond’s essay first appeared, 10.5 million
Americans were found to have diabetes. By the time Leinwand published her
results, that number had nearly doubled. Obesity rates have also climbed during
that span. Our worsening diets have made the python’s way of eating more germane.
Leinwand
has begun to investigate whether the snakes themselves get fat and sick. In the
25 years since Secor started work in Diamond’s lab, escaped or discarded
Burmese python pets have found their way into the Everglades in Florida. With
no natural predators, they have flourished on the region’s supersize supply of birds,
raccoons and opossums, as well as bobcats, deer and alligators. Leinwand wants
to know whether the Burmese python — or the Burmese-American python, really —
might be getting ill from all this food. (And if it isn’t, then what, exactly,
is protecting it?) “Their diet is different, they’re eating things that they’re
not normally eating and they’re eating way more often,” Leinwand says. “So one of the experiments that we’re working
on right now is asking, Is this a bad thing for them, or a good thing, to be
exposed to such huge amounts of food and fat?”
It’s
not clear when Leinwand’s and others’ newer projects might develop. Python
research has been very slow, especially compared with the work done on rodents.
It took Leinwand and Riquelme several years just to make sure that antibodies they
used for their mouse research would work the same for snakes. And while a round
of lab mice can be bred in a matter of weeks, new python hatchlings arrive only
once per year.
That
glacial pace makes the work on snakes “a very high-risk, high-reward project,”
Choudhary says. Last year, he and Secor and a third researcher, Bridget Wagner,
secured a grant of $2.5 million from the National Institutes of Health, from a special
fund for “innovative, unconventional, paradigm shifting research projects.”
They have started by setting up what Choudhary calls a “snake infrastructure,”
so they can
rear
and analyze the pythons as efficiently as possible.
The work is on the verge of
getting easier for another reason too. When Leinwand and Riquelme started on
the Burmese python, the animal’s genetic code had not been mapped. They did
their best by comparing bits of python RNA to published genomes of the chicken
and the lizard. But just before they published their research, a group of snake
biologists put out a first draft of the sequence for the Burmese python. Now Leinwand’s
lab, as well as Choudhary’s and Stylopoulos’s, has a detailed atlas of the
python’s DNA. That means they can feed a Burmese python in the lab and then use
a simple probe to analyze tissue from its heart, pancreas or small intestine to
see which genes are tuning up or down within each cell. Choudhary has been
using this approach to figure out the genes behind the beneficial changes to a
python’s beta cells. These might be useful targets for a future diabetes drug,
he says.
If all this work on snakes
continues to expand, the python may one day be as central to our understanding
of disease — or at least those illnesses that stem, in part, from overeating —
as the laboratory rodent. Eventually, in some respects, it might even overtake
the mouse. Perhaps that would be fitting. As she raises the baby Burmese
pythons for her lab, Leinwand has them on an all-mouse diet.
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