The patient’s seizures were getting more severe and increasingly frequent. One or two per month grew to several each week. Each burst of uncontrolled electrical activity sent shock waves through his injured brain, causing tremors and confusion. Unable to eat, his body weight dropped by nearly one-third in a few months. His health was deteriorating fast.
In October 2020, the patient—a seven-year-old sea lion named Cronutt—underwent an experimental brain surgery that involved transplanting healthy pig neurons into his damaged hippocampus. Now, more than a year since the treatment, Cronutt is seizure-free, says Scott Baraban, a neuroscientist at the University of California, San Francisco, who led the effort. Cronutt’s appetite and weight have returned to normal, he’s more social, and he’s learning new things, like how to tell left from right. Researchers say the procedure paves the way for a new strategy to treat epilepsy, but it will likely be years before the technique is attempted in people.
About 1.2 percent of the U.S. population—3.4 million people—have active epilepsy. Some forms of epilepsy are debilitating, causing a person to shake uncontrollably and become unaware of their surroundings. There are more than 30 anti-seizure medications on the market, but roughly one third of patients don’t respond to them.
Karen Wilcox, a professor of pharmacology and toxicology at the University of Utah who wasn’t involved in the transplant, thinks the cell therapy developed by Baraban and his team could one day offer hope to epilepsy patients for whom current drugs don’t work.
“It’s a very promising approach,” says Wilcox, whose research focuses on epilepsy.
The cells Cronutt received are meant to suppress the abnormal brain activity that gives rise to seizures. Many current epilepsy drugs work in the same way, but they can cause a host of unpleasant and mood-altering side effects because they affect the whole brain.
“If you can really focus the application of the therapy right where the seizures are generated, you could spare the other parts of the brain from some of the side effects that we see with taking medications,” she says.
Lethargic and disoriented, Cronutt was taken in by Six Flags Discovery Kingdom in Vallejo, California, after getting stranded on land in 2017. His brain was damaged from exposure to domoic acid, a neurotoxin produced by algae and bacteria blooms found along the Northern California coast. The toxin accumulates in the small fish and shellfish that sea lions and other marine mammals eat. In 2014, Stanford researchers determined that exposure to domoic acid in sea lions causes brain damage similar to that found in humans with temporal lobe epilepsy, the most common form of the disease.
The same year, a record 244 cases of domoic acid poisoning were documented in sea lions during the height of “the Blob,” the warm water event that stretched along the Pacific West Coast from Mexico to Alaska. In recent years, 100 or more sea lions are found sickened with domoic acid poisoning annually, according to the Marine Mammal Center in Sausalito, California. Many die from the effects. Domoic acid poisoning also has been reported in seals, sea otters, and whales.
“We’re seeing harmful algal blooms becoming larger and more persistent,” says Claire Simeone, Cronutt’s long-time veterinarian and previously the hospital director at the Marine Mammal Center. “They’re not going away.” Warmer waters caused by climate change and increased runoff from fertilizer, stormwater, and wastewater are the major factors driving the proliferation of these blooms.
By September 2020, Cronutt’s condition was dire. Simeone, along with the rest of Cronutt’s care team at Six Flags, tried every drug they could think of: appetite stimulants, pain medication, steroids, anticonvulsants. But nothing helped.
“We were running out of time with him and we needed to do something,” she says.
In all likelihood, Cronutt would have to be euthanized. As a last-ditch effort, Simeon reached out to Baraban, who for years has been working on an epilepsy therapy that involves transplanting early-stage brain cells harvested from pig embryos. In mice, transplants of the cells have been effective at stopping seizures and restoring diminished cognitive and physical abilities. Maybe the same technique could be tried for Cronutt, Simeone thought.
Baraban agreed to help, and in a matter of weeks they had assembled a team of neurosurgeons, researchers and veterinarians to help with the procedure.
On the morning of Oct. 6, 2020, the 18-person team met outside an animal hospital near San Francisco. COVID-19 protocols meant that only a handful of people could be in the clinic’s operating room, so Cronutt was sedated on a gurney in the parking lot. Mariana Casalia, a neuroscientist in Baraban’s lab, had brought the pig cells needed for Cronutt’s surgery. For decades, scientists have been investigating whether pigs can be organ donors for people who need lifesaving transplants. Pig organs, including the brain, are similar in size and function to human ones.
Casalia has developed a technique for extracting special precursor neurons—called medial ganglionic eminence cells—from pig embryos. During brain development, these cells migrate to the hippocampus and become inhibitory neurons, which counteract hyperactivity in the brain, maintaining a delicate balance of electrical activity. In the brains of people with epilepsy, many of these inhibitory neurons are lost or damaged.
“These cells, when transplanted in mice, completely cure their epilepsy,” Baraban says.
But Baraban and his team had never operated on a sea lion before, only rodents. Before injecting the pig cells into Cronutt, neurosurgeons first had to pinpoint the source of his seizures. Using MRI scans and X-rays, they examined Cronutt’s hippocampus. Embedded deep in the brain, the hippocampus is involved in learning and memory and is especially prone to seizures. There they found telltale signs of brain damage: the left side of Cronutt’s hippocampus was scarred and shrunken.
Hoping to calm the errant electrical activity in his brain, surgeons gave Cronutt four injections of around 50,000 cells each into his left hippocampus. In rodents, Baraban and his team typically do two to three injections of cells at a time. Only about 10 to 20 percent of those cells ultimately survive and integrate into the brain. The surgery, which involved making a hole in Cronutt’s skull to inject the cells, took five hours.
During the weekend leading up to his surgery, Cronutt had 11 seizures. More than a year later, his caretakers at Six Flags have yet to observe one. Simeone explains that Cronutt is closely monitored for neurologic signs indicative of a seizure, such as tremors, disorientation, lethargy or staggering. So far, Cronutt hasn’t displayed any of these symptoms. In fact, Cronutt seems to be thriving. He’s more responsive to his handlers and he’s made friends with his sea lion neighbor, Missy. Before, he would go days without eating. Now, he’s eating regularly and his weight is stable.
“I think he feels great,” says Baraban, who visits Cronutt regularly. “I could not be more pleased with his progress thus far.”
Of course, Cronutt is just one animal—albeit a much happier one now. Baraban and his team will need to carry out transplants in more sea lions to learn just how safe and effective the procedure is. Then, regulators will have to decide whether such transplants should be tried in human epilepsy patients.
Cronutt’s recovery, Baraban says, is similar to what he’s observed in mice who get transplants of the embryonic cells. In mice, the transplanted cells spread throughout the hippocampus and repair the brain circuitry that causes seizures. The cells also reduce anxiety and memory problems in the mice. Baraban suspects the cells are having the same effect in Cronutt.
At this time, Baraban and his team aren’t planning to do more brain scans on Cronutt. That would require intubating and anesthetizing him for several hours—a risky procedure. Baraban says they only have plans to obtain additional scans if Cronutt’s health declines significantly or he dies.
The procedure can’t reverse damage already done to Cronutt’s brain, but it could prevent further damage by preventing subsequent seizures. Cronutt will likely still face some mental challenges, but his caretakers are now hopeful that he could live into his 30s—the typical lifespan of a sea lion in captivity.
Baraban and Simeone are hoping to treat more sea lions in captivity that have been sickened by domoic acid so that they can track the health of the animals. If the procedure proves successful, they hope to treat sea lions at rehabilitation centers that are later released back into the wild. While more sea lions could stand to benefit from the procedure, Simeone says it’s not a long-term solution to the rise of harmful algal blooms.
Beyond marine mammals, the procedure holds promise for treating people with epilepsy for whom medication does not work.
“What the scientists did here is very important and suggests that there are alternative ways to treat epilepsy,” says Jacqueline French, chief scientific officer of the Epilepsy Foundation and a neurologist at New York University.
Current epilepsy treatments
For some epilepsy patients, surgery is another option. Neurosurgeons can either implant devices that act as pacemakers for the brain or remove an area of the brain where seizures occur. But these surgeries are invasive and carry the risk of behavioral and cognitive side effects.
A transplant of pig cells isn’t exactly without risk though. A chief concern is that the immune system could reject the transplanted cells, causing swelling and further damage in the brain.
Immune rejection has been a major hurdle in the effort to use pig organs in people who are waiting for transplants. In a recent advance, researchers at an NYU hospital overcame this immediate rejection when they attached a kidney from a genetically engineered pig to the body of a woman who was braindead and on a ventilator. The pig used in the procedure was engineered to lack a gene that causes swift immune rejection. The kidney functioned for two days, the duration of the experiment. A second experiment performed in November showed similar results.
In the brain, immune responses and inflammation are highly controlled, making rejection less likely. Cronutt took a course of immunosuppressant drugs for a short time during and after the surgery to make sure his body didn’t reject the cells. To avoid immune rejection in people, Baraban says pig embryos engineered to lack certain immune genes could be used as a safer source of the neural cells.
Whether pigs are the best source of cells, however, remains to be seen. Epilepsy researchers have long surmised that fetal cells from human embryos may be able to ease seizures in the brain. But obtaining these cells from fetal tissue is ethically fraught, which is why researchers are turning to another potential source: the patient.
Scientists at Harvard and elsewhere are reprogramming human skin cells into an embryonic-like state and then coaxing them into early-stage inhibitory neurons. These reprogrammed cells have been shown to ameliorate seizures in mice. A San Francisco-based biotech company, Neurona Therapeutics, is growing these types of stem cells in hopes of eventually treating patients with a variety of brain disorders.
Derek Southwell, a neurosurgeon at Duke University, is cautious about calling Cronutt’s recovery a cure. For one, it’s difficult to measure seizure activity in human patients, let alone in animals. It’s also unclear how many of the transplanted cells have survived and integrated into Cronutt’s brain.
Pig brain cells have previously been transplanted in Parkinson’s patients, first in the late 1990s and again in 2017, with lackluster results. One possible reason is that the patients enrolled in the trials were too far advanced in their disease, says Roger Barker, a neurologist at the University of Cambridge. Another is that too many of the cells died off before integrating into the brain.
The exact cell types, number of cells needed, and placement of the injections are all details that will need to be worked out before the procedure can be tried in people with epilepsy. Too many cells could lead to the formation of tumors in the brain.
“There is going to have to be a lot of experimentation to make sure that what you're doing is going to help and not harm,” French says.
Editor's Note: This article has been updated to clarify the use of embryonic cells in mouse studies.