Each year, about 10,000 babies in the United States are born with critical heart defects that may require surgery to fix, often fairly soon after birth. In these infants, the two major arteries of the heart could have swapped positions or the left side of the heart may be far too small to pump properly.
Doctors and scientists at Boston Children's Hospital have been working to improve those odds using a creative idea: replacing the fuel source in damaged heart cells to help them bounce back from injury. That fuel source is mitochondria, the "batteries" found in most cells in the body. In this case, the mitochondria are isolated from an abdominal muscle and then injected directly into the heart.
James McCully (left) and his colleague Dr. Sitaram Emani of Boston Children's Hospital have developed a technique to transplant mitochondria into the hearts of children recovering from surgery. (Image credit: Courtesy of Boston Children's Hospital)
"The first one or two for me were pretty nerve-wracking," said James McCully, a research associate at Boston Children's Hospital and associate professor of surgery at Harvard Medical School whose pioneering work paved the way for these procedures. His team isolates and purifies the mitochondria that a surgical team then transplants into patients.
Baby heart infusions are just one example of the promise of "mitochondrial transplantation." Following McCully's lead, other scientists have started investigating whether infusions of mitochondria could revitalize donated organs destined for transplantation, help stroke patients recover, and treat kids with devastating diseases. This work is still highly experimental, and its route to regulatory approval remains unclear.
The human body contains tens of trillions of cells, most of which contain hundreds to thousands of mitochondria. Following some back-of-the-envelope math, that means the total potential energy in each person "is the equivalent of a lightning bolt," Douglas Wallace, director of the Center for Mitochondrial and Epigenomic Medicine at Children's Hospital of Philadelphia, told attendees last year at a symposium on mitochondrial medicine.
A leading theory posits that mitochondria evolved from simple microbes. And today, they still retain distinctly microbial features. (Image credit: Behnoush Hajan/Science Photo Library via Getty Images)
Dr. Jon Brestoff, an associate professor of pathology and immunology and director of the Immunometabolism Initiative at the Washington University School of Medicine in St. Louis, describes this ability to move from cell to cell as a relic of mitochondria's bacterial past. "It is hypothesized that mitochondria transfer has likely been retained throughout evolution of eukaryotic life," the huge domain of life that includes all plants, fungi, protists and animals, Brestoff told Live Science.
Mitochondria retained bacteria-like features, such as having their own DNA and membranes. But in 2006, researchers reported that mitochondria can also exit their "host" and survive the journey into a totally different cell — a phenomenon known as mitochondrial transfer.
The 2006 study showed that cells that had been depleted of their own mitochondrial DNA could obtain healthy mitochondria from neighboring cells. After receiving other cells' powerhouses, the once-depleted cells were able to conduct energetically costly processes, such as cell division.
Jon Brestoff is studying the use of mitochondrial transplantation for genetic diseases that impair the function of these cellular powerhouses. (Image credit: Courtesy of Jon Brestoff)
Because those first experiments were done in lab dishes, initially, "it was unclear whether or not it was really biologically meaningful or relevant," Brestoff said. But within the past five to six years, improved tools and techniques have enabled scientists to watch mitochondrial transfer unfold within living organisms, he noted. It's been shown that cells of the same type can exchange mitochondria, as can cells of different types.
"That procedure," Brestoff said, "is rooted in the biology of mitochondria transfer."
"The heart came back"
Since then, these complementary lines of research — the foundational biology and the preclinical investigation — have continued in parallel. McCully's preclinical work helped to inspire other labs to explore the use of mitochondrial transplantation for a range of medical applications, including organ transplants and treatments for stroke and genetic disease.
McCully's road to mitochondrial transplantation started in the 1980s. As a molecular biologist, he wanted to uncover mechanisms that protect the heart from damage, particularly when the organ's oxygen supply is compromised — such as during ischemia, or low blood flow. With an eye on older adults, he wanted to spare patients' heart tissue from the damage and cell death that can occur before and during heart surgery.
James McCully, a research associate at Boston Children's Hospital
They found that ischemia triggers damage and dysfunction in mitochondria that persists even after the heart's oxygen supply is restored; this dysfunction ultimately kills cells. The damaged mitochondria swell up, leak their innards, and set off a chain reaction that causes a cell to self-destruct.
So, logically, they started looking directly at mitochondria, studying what the organelle looks like before, during and after ischemia. One day, while isolating mitochondria from a laboratory pig, the animal developed fibrillation, meaning its heart began beating very quickly, with an uncoordinated and rapid rhythm. The team tried to reset the heart's rhythm with electric shocks and medications, to no avail. But McCully had an idea.
"And the heart came back — started beating normally. It changed color, went back to a pink hue, and just kept on going," he said.
Live ScienceEmani, S.M., et al. (2017) The Journal of Thoracic and Cardiovascular Surgery.Emani, S.M., et al. (2017) The Journal of Thoracic and Cardiovascular Surgery.After seeing McCully's results in various animal experiments, including more with pigs, Dr. Pedro del Nido, chair of cardiac surgery at Boston Children's and a professor of child surgery at Harvard Medical School, approached him about testing the technique in human patients — namely, in babies on a life-support machine known as ECMO.
They sought permission from the Food and Drug Administration (FDA) to test the procedure and cleared the required regulatory hurdles. The process was made slightly easier because the mitochondria were coming from the patients' own bodies, not from a donor, McCully noted. Once given the go-ahead, they set forth with the help of Dr. Sitaram Emani, a heart surgeon at Boston Children's. The first procedure took place in 2014.
"It's great to be able to see something you've done and worked on for so many years actually provide a benefit," McCully said.
Dr. Giuseppe Orlando thinks that mitochondrial transplantation could serve as a potential method for fortifying donated organs that are intended for transplant patients. (Image credit: Courtesy of Dr. Giuseppe Orlando)
To Orlando, mitochondria represent a potential way to make donated kidneys work better. "In transplant medicine, one of the major issues is making sure that the organs that are procured are well stored and kept viable until implantation," said Orlando, who is also president of the Cell Transplant and Regenerative Medicine Society.
Donated organs suffer some damage when they're unplugged from the donors' bloodstream, in part because this cuts off mitochondria's crucial oxygen supply. And then, paradoxically, the donor organs also get damaged when they're plugged into the recipient's bloodstream. That's because the fast flow of blood triggers the production of free radicals — unstable molecules that damage tissues.
"You want to make sure that the kidney wakes up faster," Orlando said. That's where mitochondria come in.
In pigs, the researchers ran experiments in which the animals' kidneys were collected and stored in the same way human kidneys commonly would be prior to being transplanted. One set of kidneys received a placebo treatment, while another got an infusion of mitochondria from the donor pigs' muscle tissue. Upon being reperfused, the treated kidneys produced fewer free radicals and accrued less damage than the untreated kidneys did, the team reported in 2023.
Orlando thinks that mitochondrial transplanations could help organs "wake up" faster after being transferred from an organ donor to a recipient. (Image credit: PIERRE-PHILIPPE MARCOU via Getty Images)
Some groups are exploring mitochondrial transplantation in other donor organs, like the lungs — specifically, lungs treated with "ex vivo perfusion" (EVP), an emerging approach that helps retain the viability of transplant organs. Rather than placing a donor organ in a cooler of ice, as is generally standard, EVP flushes an organ with nutrient- and oxygen-rich fluid, helping to maintain its function until the transplant surgery. In lab experiments with pig and human lungs, scientists found that introducing fresh mitochondria during EVP further improved the organs' function while reducing inflammation and free radicals.
In the near term, Orlando and his colleagues are continuing to study the basic science of mitochondrial transplantation for this clinical purpose; they've also been cleared to run a clinical trial with donor kidneys but need more funds to proceed.
Reaching the brain
If infusions of fresh mitochondria could help the heart, kidney and lungs, what about the brain? That's the question Dr. Melanie Walker and colleagues at the University of Washington School of Medicine (UW Medicine) want to answer.
Standard treatments can dissolve the clot but don't protect the brain, Walker said. "What about all those cells that are maybe not quite dead — they're on the verge of maybe dying? What can we do to support those guys?" she asked.
But Walker realized that a harmful downstream effect of stroke might provide an opportunity for treatment. Immediately after a stroke, the brain's protective shield — known as the blood-brain barrier — becomes more permeable than usual, allowing more substances to pass from the bloodstream into brain tissue. That might open a door for mitochondrial transplantation.
One method of treating ischemic strokes is to physically remove the clot using a procedure called a thrombectomy, which a surgeon is preparing for in the photo above. Dr. Melanie Walker and colleagues tried combining this approach with mitochondrial transplantation. (Image credit: BSIP via Getty Images)This work paved the way to the first trial using mitochondrial transplant in the human brain. The small safety trial included adult patients who'd had ischemic strokes and underwent thrombectomies — surgeries to remove the clots from their blood vessels. Four received mitochondrial transplants during their procedures and 12 did not. Mitochondria were injected near the site of each patient's stroke, into the closest branch of one of the brain's major arteries.
Future trials will also need to explore exactly where the transplanted mitochondria end up in the human brain, how well they function once inside brain cells, and how the dose and timing of the transplant affect patients' outcomes, the trial runners wrote in their report. For the moment, Walker said her team is focused on exploring the safety aspects of the approach and have no further trials planned at this time.
Mitochondrial diseases — and what's next
"Those kids have a terrible prognosis," Brestoff said. Most children with the most common form of the syndrome, whose symptoms show up by age 2, die by age 3. There is no cure for the syndrome, and treatments to relieve its symptoms are limited. So if it works, mitochondrial transplantation could fill a big gap.
I can get the mitochondria from you and put them into me, me into you — doesn't really matter.
Some companies, such as Israel-headquartered Minovia Therapeutics, are already running clinical trials with patients with other types of mitochondrial disease. Minovia is running one with children with Pearson syndrome, in which missing mitochondrial DNA hinders the function of bone marrow, which normally makes blood and immune cells.
Such a product could eliminate the step in Boston Children's heart procedure where mitochondria have to be isolated from a patient while they're waiting, open on an operating table. "Getting them from the recipient can sometimes be problematic, because the recipients are sick. All the organs, all the tissue is quite distressed," McCully noted. A standardized and safety-tested mitochondrial product would therefore be a boon for the field.
A shift to heterologous transplantation does raise concerns about immune reactions. "You have some variations in the proteins that are present in your mitochondria that I don't have," Brestoff noted. "So it is possible that our immune systems recognize heterologous mitochondria as non-self."
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Despite that hurdle, the field has advanced significantly since its start as a niche, theoretical interest and then a highly experimental procedure for the sickest newborns. Scientists are now exploring the treatment as a solution to many common conditions, from rheumatoid arthritis to chronic pain.
"It just turned out that it was revolutionary, when all of a sudden you could just give a person a kidney transplant," he said. "I think it's going to be kind of similar with mitochondria."
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