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What Miniature Lab-Grown Brains Reveal About the Effects of Covid-19

Organoids are helping scientists study the coronavirus

Close up of purple gloved fingers holding up a small test tube of clear liquid of brain organoids.
Close up of purple gloved fingers holding up a small test tube of clear liquid of brain organoids.
A test tube containing brain organoids. Photo: Center for Alternatives to Animal Testing, Johns Hopkins University

he tiny blobs of brain tissue that Thomas Hartung grows in his lab at Johns Hopkins University aren’t much to look at. Just barely visible, they are little more than squishy white specks.

Known as “mini brains,” or organoids, these minuscule structures made from stem cells contain neurons that spontaneously emit electrical activity as a real brain would. The ones Hartung grows resemble the brain of a human fetus at five months of development.

Hartung and his team are using the brain organoids to better understand SARS-CoV-2, the virus that causes Covid-19. What they’ve found so far about the brain’s susceptibility to the virus is concerning: “It’s bad news adding to a pile of bad news,” Hartung tells OneZero.

Scientists have been growing organoids for over a decade, but the current pandemic has led to a flurry of interest in using them to study the new coronavirus. Researchers are now conducting similar tests with miniature lungs, guts, and livers, as well as rubbery “organs-on-chips.”

There’s a lot scientists still don’t know about the virus, and lab animals can only tell us so much. Since many animals don’t get Covid-19 like people do, human mini organs offer a way to learn which cells the virus can infect and how infection damages the body. Plus, organoids are a faster and cheaper option than using research animals because they can be mass-produced by the hundreds or thousands in the lab. Scientists are also using mini organs as stand-ins for real ones to test potential drugs to treat Covid-19.

Before Covid-19, brain organoids helped unravel another viral mystery: why some pregnant women who got infected with the Zika virus gave birth to babies with smaller brains and heads. When scientists exposed mini brains to Zika, they found that still-developing neurons were especially susceptible to the virus.

After Hartung and his colleagues read reports of some Covid-19 patients experiencing neurological symptoms in addition to respiratory ones, they wanted to know whether SARS-CoV-2 could infect brain cells, too.

It was the job of C. Korin Bullen, a postdoctoral researcher at Johns Hopkins Medicine, to find out. To work with the dangerous live virus, she donned a protective suit, shoe covers, gloves taped at the wrists, and a hooded face shield. She then entered a biosafety lab, where she exposed the brain organoids to the coronavirus.

What she found was that the virus could infect the mini brains and, 72 hours later, it began multiplying inside them, suggesting that human brain cells are susceptible to the virus. The results were published online June 26 in the journal ALTEX: Alternatives to Animal Experimentation.

“This means the virus has the potential to infect human brain cells, which is very much in line with the many neurological symptoms seen in patients,” Hartung says.

In the Journal of the American Medical Association on April 10, Chinese researchers reported that about 36% of 214 Covid-19 patients at a Wuhan hospital had neurological symptoms in addition to respiratory ones. And a study published July 8 in the journal Brain found that neurological complications of Covid-19 can include delirium, brain inflammation, stroke, and nerve damage.

It’s not yet known how the virus causes these symptoms. It’s possible that SARS-CoV-2 can cross or at least weaken the blood-brain barrier, the protective border meant to keep toxins and pathogens out of the brain. Organoids lack this barrier, so Hartung and his team couldn’t test the virus’s ability to penetrate it. But if the virus can affect the brain, it could have implications for drug development. To effectively treat Covid-19 patients with neurological symptoms, Hartung says you might need a drug that can pass through the blood-brain barrier. Not all drugs can.

The Johns Hopkins study also raises concerns for pregnant women. Like real human brains, the mini brains contain the same receptor, called ACE2, that allows the virus to enter lung cells. While there’s no evidence yet that the virus causes miscarriage, birth defects, or developmental disorders, Hartung says the possibility can’t be ruled out yet.


make the mini brains, Hartung and his team start by taking skin cells from a healthy adult and genetically reprogramming the cells to an embryonic-like state. At this nascent stage, stem cells have the potential to turn into any other cell type in the body. To coax them into becoming brain cells, scientists feed them a specific cocktail of nutrients and growth factors. Over about eight weeks, the cells grow into clumps of brain-like tissue. At about 350 micrometers in diameter, the mini brains in Hartung’s lab are smaller than the head of a pin.

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A lab-grown organoid with brain cells (blue) that have been infected with the SARS-CoV-2 virus (red). Credit: Lena Smirnova, Center for Alternatives to Animal Testing

At the Hubrecht Institute for Developmental Biology and Stem Cell Research in the Netherlands, researchers use a different approach to create organoids. They take a small tissue sample from a person’s organ they want to replicate and separate out the adult stem cells, which are found in small numbers in adult tissue and have the ability to divide and replenish damaged or dying cells. These stem cells are then grown into tiny clumps, forming tissue. They take less time to make than the brain organoids and can be ready in a week or two. (The mini brains can’t be made this way because it’s difficult to get a hold of real human brain tissue.)

Researchers at the Hubrecht Institute used this method to make mini guts to see whether SARS-CoV-2 can directly infect cells in the intestine and replicate there.

“There was a lot of clinical data pointing to the fact that patients could show up in hospitals primarily with abdominal problems, like diarrhea or stomach aches, rather than respiratory problems,” Hans Clevers, principal investigator at the Hubrecht Institute and an organoid pioneer, tells OneZero. There’s also the fact that the cells lining the inside of the intestine are covered in ACE2 receptors.

Sure enough, Clevers and his team found that the virus easily infected the mini guts and replicated in them rapidly. The findings, published May 1 in the journal Science, could explain why one-third of Covid-19 patients experience gastrointestinal symptoms such as nausea and diarrhea, and why the virus can sometimes be detected in stool samples.

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Intestinal organoids, the right one infected with SARS-CoV-2. Credit: Joep Beumer, Hubrecht Institute

ini organs are also proving useful in the effort to speed up the search for effective Covid-19 drugs. Ya-Wen Chen, a stem cell biologist and assistant professor of medicine at the University of Southern California, is using lung organoids to test several drugs in her lab.

Two are antivirals to prevent SARS-CoV-2 from entering the cells that line the lungs. A third could potentially block a serious immune reaction seen in some Covid-19 patients called a “cytokine storm.”

Chen says an advantage of using organoids is that they’re three-dimensional, as opposed to human cell lines, which are typically flattened into two dimensions in a petri dish when used to study viruses. Another reason she likes using organoids is that they’re a more simplified model than an animal. “You eliminate the more complex environment you see in the animal. That allows you to focus on studying the effects you want to see,” she says.

Chen’s lab uses the same method as the mini brains to make lung organoids. When they grow in a dish, they form tiny versions of the branching airway structures seen in full-sized lungs. She starts using them for drug testing when they’re about the size of the eraser on a pencil.

While organoids have certain benefits over cell lines and mice, they still have limitations. Organoids are made up of one type of tissue and they don’t have structures like blood vessels, immune cells, or connective tissue that you would find in a real organ.

At the Wyss Institute for Biologically Inspired Engineering at Harvard University, scientists are taking mini organs to the next level. They have been building clear, flexible “organs-on-chips” that more closely mimic the functions of a real human lung. About the size of a computer memory stick, the chips have tiny hollow channels that contain multiple types of tissue and mimic blood flow as well as air exchange in the lungs.

In June, the Wyss Institute and U.S. Defense Advanced Research Projects Agency inked a deal worth up to $16 million over the next year to identify drugs already on the market that could be repurposed to prevent or treat Covid-19. As part of that effort, Wyss researchers are testing the effects of promising drugs on their lung and intestine chips.

A lot of drugs work on cell lines but don’t end up being effective in human clinical trials, says Donald Ingber, founding director of the Wyss Institute. All that research is lengthy and expensive. “That made us realize that we can basically use organ chips as sort of a funnel to narrow down a large number of compounds,” Ingber says. The idea is to use organs-on-chips to better predict which drugs are likely to work in people.

The Wyss Institute doesn’t have a biosafety lab designed to study deadly pathogens, so it first used a mock virus that acts like SARS-CoV-2.

Ingber and his team tested seven drugs that are already approved by the Food and Drug Administration on chips designed to mimic the human airway. The chips contain lung epithelial cells, which express high levels of the ACE2 receptor. All seven drugs looked effective against the pseudo-SARS-CoV-2 virus in cell lines, but when the drugs were all tested on the lung chips, only two effectively blocked the virus from entering cells: an antimalarial drug called amodiaquine and a breast cancer medication called toremifene.

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An organ-on-a-chip. Credit: Wyss Institute

Ingber and his colleagues posted their findings online in April, but the paper has yet to be peer-reviewed. They’re now collaborating with two other research groups that are working with the real virus in a high-level safety lab, where they are using the chips to test various drugs.

Beyond the current pandemic, mini organs could potentially be used to spot the next big threat to humans before it can wreak havoc. Clevers says once a new animal pathogen is identified and isolated, scientists could take that virus or bacteria and use it to try to infect different types of mini organs. Doing so could give researchers an idea of which emerging pathogens are potentially dangerous to humans.

Clevers says organoids can also be made from animals, so scientists could try to infect them with different pathogens to learn which animals could act as reservoirs. In May, scientists in China reported in Nature that bat organoids could be infected with SARS-CoV-2, lending evidence to the idea that the virus originated in bats.

“You could actually detect viruses before they jump,” says Clevers. “Then you could have your treatments and vaccines ready before the epidemic ever hits.”

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Staff writer at OneZero covering biotechnology. Previously, associate editor for biomedicine at MIT Technology Review.

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