Scientists have developed a research method that allows a much more detailed examination of the brain processes involved in certain neurological and mental disorders. This is achieved by growing human cortical organoids in culture and inserting them into developing rodent brains to see how they integrate and function over time. The study, funded by the National Institute of Mental Health (NIMH), part of the National Institutes of Health, appears in the journal Nature.
“This work provides a significant advance in scientists’ ability to study the cellular and circuitry underpinnings of complex disorders in the human brain. It allows organoids to be ‘hardwired’ into a more biologically relevant context and to function in ways they can’t in a petri dish,” said David Panchision, Ph.D., chief research officer. developmental and genomic neuroscience research arm of the NIMH’s Division of Basic Neuroscience and Behavioral Sciences.
Researcher Sergiu Pasca, MD, and colleagues at Stanford University, Stanford, California, have demonstrated that a cortical organoid cultured from human stem cells can be transplanted and integrated into the developing rat brain to study certain development and functional processes. The results suggest that transplanted organoids may offer a powerful tool to study processes associated with disease development.
Researchers sometimes use cortical organoids — three-dimensional cultures of human stem cells that may mirror some of the developmental processes seen in typical brains — as a model to study how certain aspects of the human brain develop and function. However, cortical organoids lack the connectivity seen in typical human brains, which limits their usefulness for understanding complex brain processes. Researchers have attempted to overcome some of these limitations by transplanting individual human neurons into adult rodent brains. Although these transplanted neurons connect to rodent brain cells, they do not fully integrate due to developmental limitations of the adult rat brain.
In this study, the team of researchers advanced the use of brain organoids for research by transplanting an intact human cortical organoid into a developing rat brain. This technique creates a unit of human tissue that can be examined and manipulated. The researchers used methods previously developed in the Pasca lab to create cortical organoids using human induced pluripotent stem cells – cells derived from adult skin cells that have been reprogrammed into a cell-like state. immature strain. They then implanted these organoids into the rat’s primary somatosensory cortex, a part of the brain involved in processing sensations.
The researchers did not detect any motor or memory abnormalities or brain activity abnormalities in the rats that received the transplanted organoid. Blood vessels from the rat’s brain successfully supported the implanted tissue, which grew over time.
To understand how well organoids might integrate into the rat somatosensory cortex, the researchers infected a cortical organoid with a viral tracer that spreads through brain cells as an indicator of functional connections. After transplanting the labeled organoid to the rat’s primary somatosensory cortex, the researchers detected the viral tracer in several brain areas, such as the ventrobasal nucleus and the somatosensory cortex. In addition, the researchers observed new connections between the thalamus and the transplanted area. These connections were activated using electrical stimulation and stimulation of the rat’s whiskers, indicating that they were receiving significant sensory input. Additionally, the researchers were able to activate human neurons in the transplanted organoid to modulate the rat’s reward-seeking behavior. The results suggest functional integration of the transplanted organoid into specific brain pathways.
Structurally and functionally, following seven to eight months of growth, the transplanted brain organoid resembled neurons in human brain tissue more than human organoids maintained in cell culture. The fact that transplanted organoids mirrored the structural and functional characteristics of human cortical neurons led researchers to wonder if they might use transplanted organoids to examine aspects of human disease processes.
“The promise of this platform is not only to identify the molecular processes that underlie the advanced maturation of human neurons in living circuits and to leverage them to improve in vitro models, but also by providing behavioral readouts for human neurons,” Dr. Pasca said.
To examine this, the researchers generated cortical organoids with cells from three participants with a rare genetic disorder associated with autism and epilepsy called Timothy syndrome and three participants with no known disease and implanted them on the rat’s brain. Both types of organoids embedded in rat somatosensory cortex, but organoids derived from patients with Timothy syndrome showed structural differences. These structural differences do not appear in organoids created from the cells of patients with Timothy syndrome and maintained in cell culture.
“These experiments suggest that this new approach can capture processes that go beyond what we can detect with current in vitro models,” said Dr. Pasca. “This is important because many of the changes that cause psychiatric illnesses are likely subtle circuit-level differences.
