WHEN THE CITY GIRL decides to study lactation, she must first learn to milk a cow. Laura Hernandez, an assistant professor of dairy science at CALS, remembers that lesson.
Her tutor that day was Jessica Cederquist, then a fellow grad student and now CALS herd manager. “People who have never milked are used to what you see in the movies,” Cederquist explains. You know the choreography: grab a teat, pull down, milk squirts into the bucket. But that technique simply squeezes milk back into the udder. And just about everybody makes the mistake. “It is a rite of passage to stand back and laugh,” she admits.
“She thought it was very funny,” Hernandez recalls. “I think that was the beginning of a very good friendship.”
The milking got a little crazier once Hernandez ramped up her inquiries into how lactation works. Her first experiments required milking two halves of the same cow, comparing milk production. Because she was pairing the front right with the back left and vice versa, she had to replumb two half milkers, using a surplus of hoses and buckets. She’d also recently had knee surgery.
“You’re already kind of crowded in there and now you’ve got her fancy contraption and all of her buckets and a big old knee brace,” says Cederquist. And it’s a waterbed stall, so every time anybody moves, the floor moves, and the buckets yaw precariously. “She’s darn near laying on the floor under the cow, trying to figure out how she’s going to get this thing to stay on.”
Hernandez is still making things unusual for Cederquist. Lactation is a delicate enough phenomenon that the typical dairy farmer puts animals who are in the late stages of pregnancy on vacation. This is exactly when Hernandez needs to poke and prod, monitor and manipulate.
The hassle seems worth the reward: Her exploration of the role of serotonin in lactation has the potential to significantly improve animal health and boost milk production. There may also be profound lessons about the role of serotonin in human health. While seratonin was once considered the miracle molecule of mental health, Hernandez is helping unravel its role in many more parts of the body.
“There is still an infinite box of things it probably does that we can’t understand,” says Hernandez. Which is all the more interesting because it’s such a simple molecule, just a modified amino acid. It’s as if a Lego block were able to control a nuclear reactor. “I really am just completely fascinated by how a modified amino acid can regulate what feels like the universe at times,” Hernandez says.
On the road between Hernandez’s hometown of El Paso, Texas, and the New Mexico State University campus in Las Cruces, a line of dairy farms stretches across the landscape. Despite her urban upbringing, the cows fascinated her. “As an athlete I was like: how does she do that?” recalls Hernandez, then a scholarship swimmer. “I just thought they were really cool animals, what they could do from a biological standpoint.”
Drawn to biology, Hernandez chose animal science over straight biology because she was more interested in working with mammals than with crabs and nematodes. But her real immersion didn’t begin until her senior year, when she transferred to New Mexico State from Iowa State University. In Ames her swimming schedule had kept her out of the lab, but that changed when she got to Las Cruces.
“I loved working in the lab,” says Hernandez. “That was where I found my home.” When she couldn’t decide between professional schools, she continued at New Mexico State to earn a master’s degree in animal science and toxicology.
In 2005 she started her doctorate at the University of Arizona with Bob Collier, a physiologist in the dairy sciences. He was interested in how genes interacted with the environment, and lactation was the ideal process to study: genetically programmed, but initiated and controlled by changes in the environment of the cow.
The year before Hernandez arrived, the small world of lactation science had been upended by the unexpected discovery that serotonin, long considered simply a neurotransmitter, also had a role in regulating lactation. Collier reached out to Nelson Horseman at the University of Cincinnati, where the discovery had been made. Horseman studied breast development, but his central interest was breast cancer. Collier offered his dairy expertise and suggested that they collaborate on expanding this discovery from the mouse to the cow.
Hernandez undertook the research for her dissertation, supervising many of the active experiments. Deeper she went, her work encompassing an intense collaboration into the complex molecular underpinnings of milk production.
After finishing her Ph.D. she began a postdoc in Horseman’s lab. One day in Cincinnati, Gerard Karsenty, a geneticist visiting from Columbia University, presented his research involving gut serotonin, calcium and bone mass. Afterward Hernandez turned to Horseman and wondered aloud: If gut serotonin had a role in bone mass, could this also help explain its role in lactation?
Nursing typically requires more calcium than diet alone can provide, and the difference comes from the mother’s bone. A nursing mouse will lose up to 20 percent of bone mass in 21 days. Human mothers can lose 6 to 10 percent of their bone mass over six months. Studies in West Africa and Korea suggest that the longer a woman breast-feeds, the lower her bone density.
It’s not surprising that serotonin might have more than one role in the body. Along with dopamine it’s the oldest known hormone, and nature loves to reuse its creations. In fact, serotonin first evolved in plants. Plants have no nervous system, so it couldn’t have been a neurotransmitter. How a simple molecule engages in complex processes is by acting as a molecular key in many different cellular locks. Scientists have now identified 20 different serotonin receptors. The mammary gland alone has five.
So how to uncover serotonin’s role in withdrawing calcium from bone? Scouring some old genetic assays, Hernandez found a likely ally: parathyroid hormone-related protein (or PTHrP). Her initial tests were so strong that she suspected her equipment was off.
But further experiments confirmed that serotonin was causing an increase in PTHrP in the mammary gland during lactation. This, in turn, was a key signal liberating calcium from bone for the mammary glands.
Hernandez’s research portfolio made her an obvious match when a position opened at CALS. As a newly hired professor in 2011, her first question was obvious: Could she leverage our knowledge of PTHrP in the dairy cow?
Lactation is hard, and one of the biggest problems faced by dairy farmers is the “transition cow,” a cow in the three weeks before and after calving. Between the physiologic stress of birth and the metabolic stress of commencing lactation, for the first 20 to 30 days of lactation the cow is expending more energy than she can take in.
Calcium complicates things, as it takes a couple of days to activate the mechanism that borrows from the bone. Sometimes that leads to a calcium deficit—or hypocalcemia, also knownas milk fever. Because calcium is critical for biological functions, assisting with everything from muscle contraction to immune function, a shortage can lead to a variety of potential health problems including ketosis, displaced abomasum and retained placenta. Gut issues can arise because the intestines aren’t contracting. Reduced immune function leaves the cows more susceptible to mastitis.
“That’s a precarious time frame for them,” Hernandez says. “If you have a calcium problem, other issues compound.”
It’s a daily concern for dairy farms. Even on a very good farm, 3 to 5 percent of the animals are going to wind up with milk fever. Scaled up to a 10,000-herd farm, that means one or two affected cows every day.
“Not every farmer is going to automatically relate to Hernandez’s deep molecular work,” says herd manager Jessica Cederquist. But put it in terms of milk fever and the transition cow, and “every dairy farmer on the planet knows what that means,” she says.
With startup money tight and a big idea, Hernandez developed an ambitious research agenda. She found a collaborator in Jimena Laporta, a graduate student fresh from Uruguay. Laporta read the plan and committed the very next day. “We were throwing all of the chips on the table and hoping for a win,” says Hernandez.
The idea was simple: Could you boost PTHrP levels with nutritional supplements? They fed rats two amino acids—5-hydroxytryptophan (abbreviated as 5-HTP) and straight tryptophan. Both are chemical precursors in the synthesis of serotonin.
They began with rats, and feeding was the easy part. The hard part? They also had to milk them. Forty-five rats. Every day. How do you milk a rat?
After knocking it out with sleeping gas, you inject a minute quantity of the hormone oxytocin. A small suction device evacuates the teats; each animal has 10. It was a time-consuming, two-person job. Hernandez and Laporta sacrificed weekends and postponed professional travel. Eventually they got the process down to about an hour and a half.
The 5-HTP worked. Then they confirmed that it works in the cow via IV infusion. Now the lab is working on developing a cow feed that accomplishes the same thing.
Meanwhile, on the molecular level they were focusing on how the serotonin was actually affecting the mammary gland and how it translated into the chemical signals that drive bone resorption. In addition to the PTHrP they identified a gene—already nicknamed sonic hedgehog—as another link in the chain in collaboration with researchers Chad Vezina and Robert Lipinski at the UW–Madison School of Veterinary Medicine.
“It’s a very big picture of a very small molecule,” says Laporta, now teaching at the University of Florida. “Nobody knew that serotonin could do all these things. I think we opened a black box.”
Repeat: lactation is hard. Hernandez became a mother in the first year of her professorship, and nursing was as fulfilling as it was excruciating. She was lactating, she was teaching about lactation, she was manipulating lactation. Under the grueling stress of a new research program she took only nine days of maternity leave.
One day in mid-February her husband came home to find Hernandez crying on the bathroom floor. She couldn’t find time to pump, and her hair was falling out. He suggested it might be time to stop nursing. She’d made it seven months under a colossal workload. They still had some milk stored to facilitate transition to the bottle. “But I want to make it a year,” Hernandez objected. “I’m a lactation biologist! I must!”
“It was so hard,” she reiterates. “It’s made me even more of an advocate for helping women after they give birth. That’s where my biggest interest is: The mother’s ability to deal with lactation and to do so healthily for herself while also taking care of her baby.”
And so Hernandez has forged into human health. As the role of serotonin beyond brain chemistry continues to unfold, obvious questions arise. Selective serotonin reuptake inhibitors, or SSRIs, now dominate the antidepressant market and include such household names as Prozac, Paxil and Zoloft. Among their side effects is a decrease in bone density. Nursing also decreases bone density. With 12 percent of pregnant women taking SSRIs, does the combination of SSRIs and nursing set these women up for severe bone health issues later in life?
Most studies that looked at nursing and SSRIs focused on the infant. “Almost nothing out there looks at the long-term implications for the mother,” reports Sam Weaver, a third-year Ph.D. student in Hernandez’s lab. Weaver began as an undergraduate in the lab, assisting Laporta with her milking. Now Weaver supervises her own mouse dairy as she tries to untangle the precise impact of SSRIs on lactation and the health of the mother.
Weaver harvests more than milk. The mice are dissected with precise determination as blood, mammary glands, kidneys, intestines and bone tissue are examined for health and their reactivity to serotonin. Their femur bones are sent off to a collaborator in Boston for specialized imaging.
“Can we somehow help women breast-feed but also stay on their medication, and help them avoid some of these long-term bone issues?” asks Hernandez. She hopes to begin working with human populations soon.
Now that the lab has characterized the complexity of serotonin in lactation, the team is trying to get a handle on its role as one of the body’s master regulators. Only about 2 percent of serotonin actually resides in the brain; the vast majority circulates throughout the rest of the body. “We’re finding it popping up in all sorts of places,” says Weaver.
A newer project is working on yet another serotonin-lactation connection. Obese women tend to have higher serotonin levels—and they also have a harder time initiating nursing. This suggests yet another crucial role for serotonin as a regulator of energy balance in the body. By unlocking its role, they hope to find a way to make nursing easier for these mothers.
The legacy of Wisconsin is so milk-soaked it can be hard to remember that lactation still holds mystery and marvel. It’s a unique biological process that has given up its secrets slowly, and there is still much to learn. Experiments with a wide variety of mammals have shown that as long as you keep removing milk, the gland will keep making it.
Though she’s unlocked some of the secrets behind this apparent superpower, Hernandez remains entranced: “It just fascinates me that it can continue to do that.”
It’s not a stretch to call lactation one of the more significant developments in the evolution of life on this planet. The expanded ability to feed our young has allowed mammals to adapt to a wide array of variations in our environment. “Keep the baby alive,” says Hernandez. “I think it ties back to that, making us better mothers.” Our human accomplishments are stamped with an indelible mammalian signature.
Hernandez’s peculiar dairy, with its few hundred mice and few dozen patient cows, keeps producing under the labors of a handful of motivated students. “Sometimes it’s overwhelming, and it feels like we’re not getting anywhere and we’re not going to get anywhere,” Hernandez says. “Because with every answer comes another question.”
Even as she continues her fine-scale investigations, Hernandez hopes that young farmers can go back to their dairies and incorporate some wonder into our conversations about animal agriculture.
As Hernandez and dairy farmers know, when it comes to a cow’s well-being, milk is a marker.
“If cows are not being fed properly, or taken care of properly or housed properly, they are not going to make a lot of milk,” Hernandez says. “That’s a basic mammalian response. That should tell you something about the welfare of the animals.”
