Put human stem cells in a laboratory dish with the right nutrients and they do their best to form a small brain. They fail, but you get an organoid: a semi-organized clump of cells. Organoids have become a powerful tool for studying brain development and disease, but researchers assumed that these microscopic patches only reflect the prenatal development of the brain – its earliest and simplest stages. A study today reveals that with enough time, organoid cells can take on some of the genetic signatures that brain cells show after birth, potentially extending the range of disorders and developmental stages they can regenerate.
“Things that I, before I saw this, would have said you can”not do with organoids … in fact, maybe you can, ”says Madeline Lancaster, a developmental geneticist at the Medical Research Council”s Laboratory of Molecular Biology. For example, Lancaster was not optimistic about using organoids to study schizophrenia, which is suspected to appear in the brain after birth when neural communication becomes more complex. But she now wonders if cells from a person with this disorder – once “reprogrammed” to a primitive stem cell status and the lure to mature into a brain organoid – could reveal important cellular differences underlying the condition.
Stanford University neurobiologist Sergiu Pașca has been making organoids in the brain for about 10 years, and his team has learned that some of these tissue blobs can thrive in a bowl for years. In the new study, they teamed up with neurogeneticist Daniel Geschwind and colleagues at the University of California, Los Angeles (UCLA) to analyze how blobs changed over their lifetime.
The researchers exposed human stem cells to a specific set of growth-promoting nutrients to create spherical organoids that contain neurons and other cell types found in the outer layers of the brain. They periodically removed cells to sequence their RNA, indicating which genes are active in producing proteins. They then compared this gene expression with a database of RNA from cells of human brains of different ages. They noted that when an organoid became 250 to 300 days old – about 9 months –its gene expression shifted to look more like cells from human brains shortly after birth. The cells’ methylation patterns – chemical tags that can be placed on DNA and affect gene activity – also corresponded to increasingly mature human brain cells as the organoids got older, the team reports today in Nature Neuroscience.
The researchers documented other signals of maturity in their organoids. Around the time of birth, some brain cells gradually shift to make more of one variant of one protein and less of another. A component of a brain cell receptor called NMDA, the key to neuronal communication, is among the proteins that change shape. And organoid cells, like their developing brain counterparts, got the NMDA switch.
The results do not mean that the cloth itself can be compared to a postnatal brain, warns Pașca. Its electrical activity does not match that of a mature brain, for example, and the cell clot lacks key functions, including blood vessels, immune cells, and sensory inputs. Yet it is striking that even under the unnatural conditions of a laboratory dish, “the cells just know how to move on, ”says Pașca.
Organoid cells and true brain cells may not mature into perfect lockstep, notes arna Bhaduri, a developmental neurobiologist at UCLA who was not involved in the new work. In a previous study, she and her colleagues found that organoid cells showed important genetic differences from fetal brain cells along with signs of metabolic stress. She says it is reassuring that the main changes seen at birth in the new study appear to fluctuate in an organoid just as researchers would expect – around 9 months.
Pașca’s team also looked at the expression of genes associated with brain diseases, including autism, schizophrenia, epilepsy and Alzheimer’s disease. The researchers identified clusters of these genes whose activity increased and decreased in steps and reached its highest expression at the same time. The coats of arms could indicate when these genes are most relevant for brain development – and at what point an organoid may be most useful for modeling a given disorder.
Now that it is clear, the cells in an organoid can go through some of the normal developmental routines of the human brain after birth, Pașca”s team explores ways to “push [the organoids] back and forth in time to get the right period for a disease model, ”he says. It could enable his group and others to study brain diseases in mature organoids without caring for cells for years.