Stanford Researchers Develop New Model to Study Brain Connectivity

Researchers often use cortical organoids to investigate aspects of the human brain, but these models often don't have the same connectivity in vitro as those observed in typical human brains. To address these translational challenges within the literature, scientists at Stanford University transplanted and integrated cortical organoid cultures from human stem cells in rat brains. This allowed them to study specific developmental and functional processes, with their findings, published in the journal Nature, suggesting that transplanted organoids may be a powerful tool to examine disease development.

"This work provides a significant advance in the ability of scientists to study the cellular and circuit underpinnings of complex human brain disorders. It allows organoids to get 'wired' in a more biologically relevant context and function in ways they can't do in a petri dish," says David Panchision, Ph.D., chief of the Developmental and Genomic Neuroscience Research Branch in the Division of Neuroscience and Basic Behavioral Science at NIMH.

While there have been studies that attempt to transplant individual human neurons into adult brains, they struggle to achieve full integration due to the developmental limitations of the adult rat brain. However, the team used induced pluripotent stem cells (iPSCs) to transplant an intact human cortical organoid into a developing rat brain. This technique, initially pioneered in senior author Sergiu P. Pasca's lab, creates a unit of human tissue to be examined and manipulated over time.

To better understand how these organoids integrate into the rat somatosensory cortex, the scientists infected a cortical organoid with a viral tracer to spread throughout brain cells as a connectivity indicator. Post-implantation, the team observed the tracer in multiple brain areas, like the ventrobasal nucleus and thalamus. The connections were activated using electrical and whisker stimulations, suggesting that these connections were receiving meaningful sensory input. The team also activated human neurons within the transplanted organoid to modulate the rat's reward-seeking behavior, suggesting functional integration of the organoid into specific brain pathways.

After the researchers implanted these organoids into the rats' primary somatosensory cortex, they did not detect any motor or memory abnormalities post-transplant. Additionally, the team utilized MRI to assess organoid growth across multiple stem cell lines and animals, finding that the vasculature within the rats' brains supported the implanted tissue.

Structurally and functionally, the transplanted organoid resembled neurons from human brain tissue more similarly than human organoids in culture after approximately eight months. These findings suggest that transplanted organoids may help study specific parts of human disease processes since they resemble human cortical neurons' anatomical and functional characteristics.

"The promise of this platform is not only in identifying what molecular processes underlie the advanced maturation of human neurons in living circuits and leveraging it to improve conventional in vitro models, but also in providing behavioral readouts for human neurons," says Dr. Pasca, M.D., researcher at Stanford University.

To test this, the researchers created cortical organoids from the cells of three individuals with Timothy syndrome, a rare genetic illness linked to autism and epilepsy, and three control participants. Then, they implanted these organoids into the rat's brain. While both types of organoids were incorporated into the rat somatosensory cortex, the team found that the organoids made from Timothy Syndrome patients contained structural abnormalities. Such structural variations did not appear in organoids made from Timothy Syndrome patients' cells and kept in cell culture.

"These experiments suggest that this novel approach can capture processes that go beyond what we can detect with current in vitro models," states Dr. Pasca. "This is important because many of the changes that cause psychiatric disease are likely subtle differences at the circuit level."