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Pushing Boundaries, These Studies Use Human Neuronal Engineering to Gain Insights into Disease Phenotypes

Article In Brief

Two new cellular models provide insights into disease phenotypes and neuronal behavior. One found that human stem cell-derived cortical organoids showed characteristics of a disease, Timothy syndrome, that were previously unknown. The other found that human and mouse neuronal cells in a computer chip can be taught to perform goal-oriented tasks when provided with electrophysiological sensory input and feedback.

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A transplanted human organoid labeled with a fluorescent protein in a section of the rat brain.

Two new studies that push the boundaries of human neuronal engineering raise hopes of future breakthroughs and potential ethical concerns.

The first, published in Nature on Oct. 6, found that human cortical organoids—3D neuronal cell cultures derived from human induced pluripotent stem cells—matured and integrated into newborn rat brains when transplanted into the somatosensory cortex, extending axons and engaging circuits that control behavior. Previous studies using adult rodents did not detect such extensive integration of the organoids.

When the researchers used human neurons derived from individuals with Timothy syndrome, a rare disorder that affects the heart and other body organs, they observed defects that had not been seen when organoids remained in vitro.

“We are using this technique to study several neurological and psychiatric conditions, mostly of developmental origin,” said the first author of the paper, Sergiu P. Pasca, MD, professor of psychiatry and director of the Human Brain Organogenesis Program at Stanford University.

For now, he told Neurology Today, concerns that the implantation of a human cortical organoid could result in improved intelligence in the rats remains unfounded. After following the animals for nine months or so, he said, “we tested the rats in various [cognitive] tasks but found no differences.”

Computer Chip, Video Games

The second study placed both human and mouse neurons onto a computer chip and then stimulated them electro-physiologically in an attempt to teach them to play the arcade game “Pong.” When provided with feedback, the neurons appeared to begin learning “within five minutes of real-time gameplay,” the paper published Sept. 20 in Neuron reported.

“We used the word ‘sentience’ to describe the behavior we were seeing,” said the first author of the paper, Brett Kagan, PhD, chief scientific officer at Cortical Labs in Melbourne, Australia. “The cells were able to take in outside information, do some internal processing, and then output a behavior that affected their environment.”

The study offers a potentially useful new approach to studying neurons in vitro, said Fred “Rusty” Gage, PhD, president of the Salk Institute for Biological Studies and the Adler Professor in the laboratory of genetics at the Salk Institute, who was not involved with the study.

“It's surprising, but it takes the tissue-on-a-chip technology a step forward, to see how much information you can gain with a closed-loop feedback system,” Dr. Gage said. “This is a very novel and important study. I'm certainly going to think about incorporating this into our own work.”

Use of Implanted Organoids

Using athymic rats in order to prevent rejection of the human cells, Dr. Pasca and colleagues at Stanford implanted the organoids at five days. As the animals grew, the human stem cell-derived cortical organoids integrated into sensory and motivation-related circuits, they found.

“MRI reveals post-transplantation organoid growth across multiple stem cell lines and animals,” the paper reported. “Single-nucleus profiling shows progression of corticogenesis and the emergence of activity-dependent transcriptional programs.”

The transplanted neurons displayed such complex and mature development, Dr. Pasca and colleagues reported, that they could discover previously unseen defects in neurons derived from individuals with Timothy syndrome.

“We observed an upregulation of activity-dependent genes in neurons that were transplanted versus neurons maintained in vitro,” Dr. Pasca told Neurology Today. The abnormal upregulation, he said, was seen in vivo “but not under conventional in vitro conditions.”

The transplanted organoids received thalamocortical and corticocortical inputs, “and in vivo recordings of neural activity demonstrate that these inputs can produce sensory responses in human cells,” the paper reported.

Optogenetic activation of the human cortical neurons, the study found, drove reward-seeking behavior in the rats. “We anticipate that this approach will be useful for detecting circuit-level phenotypes in patient-derived cells that cannot otherwise be uncovered,” the authors concluded.

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“The organoids may never develop the same maturity and functionality as in the human brain. We need to be careful in saying that these organoid neurons faithfully recapitulate how a human neuron in that position would behave.”—DR. SULAYMAN DIB-HAJJ

‘Dish Brain’ Neurons

The Neuron paper led by Dr. Kagan called its closed-loop system “DishBrain.” Researchers used approximately 800,000 embryonic neurons from mice and humans, which then slowly projected axons across the array as they grew.

To “teach” the neuronal array how to play Pong, Dr. Kagan and colleagues exposed them to electrodes on the left or right as a way to tell them where the “ball” was. The electrodes could then read their activity and give the neurons feedback, enabling them to learn how to return the ball with increasing accuracy. Arrays that did not receive feedback did not learn.

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“Its surprising, but it takes the tissue-on-a-chip technology a step forward, to see how much information you can gain with a closed-loop feedback system. This is a very novel and important study. Im certainly going to think about incorporating this into our own work.”—DR. FRED “RUSTY” GAGE

“What we're building is a platform,” Dr. Kagan said. “Everything you would need to replicate our findings is in the paper, other than the source code. We're making the platform available to people who want to use it at very low fees.”

Link Up for More Information

• Revah O, Gore F, Kelley KW, et al. Maturation and circuit integration of transplanted human cortical organoids https://www.nature.com/articles/s41586-022-05277-w. Nature 2022; 610(7931):319–326.
• Kagan BJ, Kitchen AC, Tran NT, et al. In vitro neurons learn and exhibit sentience when embodied in a simulated game-world https://www.sciencedirect.com/science/article/pii/S0896627322008066. Neuron 2022:S0896-6273(22)00806–6.