Brain cell clusters, grown in lab for more than a year, mirror changes in a newborn’s brain

Science

Put human stem cells in a lab dish with the right nutrients, and theyll do their best to form a little brain. Theyll fail, but youll get an organoid: a semiorganized clump of cells. Organoids have become a powerful tool for studying brain development and disease, but researchers assumedthese microscopic blobs only mirrora brains prenatal developmentits 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 display after birth, potentially expanding the range of disorders and developmental stages they can recreate.

Things that, before I saw this paper, I would have said you cant do with organoids actually, maybe you can, says Madeline Lancaster, a developmental geneticist at the Medical Research Councils Laboratory of Molecular Biology. For example, Lancaster wasnt optimistic about using organoids to study schizophrenia, which is suspected to emerge in the brain after birth, once neural communication becomes more complex. But she now wonders whether cells from a person with this disorderonce reprogrammed to a primitive, stem cell state and coaxed to mature within a brain organoidcould reveal important cellular differences underlying the condition.

Stanford University neurobiologist Sergiu Paca has been making brain organoids for about 10 years, and his team has learned that some of these tissue blobs can thrive in a dish 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 the blobs changed over their life spans.

The researchers exposed human stem cells to a specific set of growth-promoting nutrients to create spherical organoids containing neurons and other cell types found in the outer layers of the brain. They periodically removed cells to sequence their RNA, which indicates which genes are active in making proteins. Then they compared this gene expression with a database of RNA from cells of human brains of different ages. They noticed that when an organoid reached 250 to 300 days oldroughly 9 months its gene expression shifted to more closely resemble that of cells from human brains soon after birth. The cells patterns of methylationchemical tags that can affix to DNA and influence gene activityalso corresponded to increasingly mature human brain cells as the organoids aged, 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 a protein and less of another. A component of a brain cell receptor called NMDA, key to neuronal communication, is among the proteins that switch forms. And organoid cells, just like their counterparts in the developing brain, made the NMDA switch.

The findings dont mean the blob itself is comparable to a postnatal brain, Paca cautions. Its electrical activity doesnt match that of a mature brain, for example, and the clump of cells lacks key features, including blood vessels, immune cells, and sensory inputs. Yet whats striking is that, even in the unnatural conditions of a lab dish, the cells just know how to progress, Paca says.

Organoid cells and real brain cells might not mature in perfect lockstep, notes Aparna Bhaduri, a developmental neurobiologist at UCLA who was not involved in the new work. In a previous study, she and her colleagues from fetal brain cells, along with signs of metabolic stress. She says its reassuring that in the new study, key changes seen at birth seem to happen in an organoid right when scientists would expectat about 9 months.

Pacas team also looked at the expression of genes associated with brain disorders, including autism, schizophrenia, epilepsy, and Alzheimers disease. The scientists identified clusters of these genes whose activity rose and fell in step, reaching their peak expression at the same time. The crests could indicate when those genes are most relevant to brain developmentand at what time point an organoid might be most useful for modeling a given disorder.

Now that its clear the cells of an organoid can walk through some of the human brains normal postbirth developmental routines, Pacas team is exploring ways of pushing [the organoids] back and forth in time to get the right period for a disease model, he says. That could allow his group and others to study brain diseases in mature organoids without babysitting cells for years on end.