Protein inhibitor normalizes neuronal migration in organoid mannequin of autism | Spectrum
On the move: Neurons from month-old organoids with 16p11.2 deletions (right) migrate more slowly than those from control organoids (left).
Inhibiting a protein that helps cells move or change shape prevents atypical neural migration in 3D clusters of brain cells that carry genetic variants associated with autism, according to a new study. The cell models known as organoids also mimic differences in head size seen in some people with the variants.
The researchers made the organoids using reprogrammed cells from humans that had deletions or duplicates of a 29-gene stretch from chromosome 16 called 16p11.2. Both variants are strongly linked to autism and head size differences: people with deletions tend to have an atypically large head, called macrocephaly, and those with duplications have an atypically small head, called microcephaly.
Mice with 16p variants have not consistently shown differences in head size, and so the organoids could be a better model for studying this trait, says Hongjun Song, a professor of neuroscience at the University of Pennsylvania who was not involved in the work.
“It’s really fascinating in the human system that we can see that,” says Song. “This is really a nice start. It shows why we really have to examine people when it comes to human diseases. “
The organoids could help uncover how 16p11.2 variants affect the earliest stages of brain development, researchers say. They could also come in handy in identifying treatments, says lead investigator Lilia Iakoucheva, an associate professor of psychiatry at the University of California, San Diego.
“Of course, the bigger goal was maybe to find some drug targets to help the children,” says Iakoucheva. “We thought that a patient-derived model was ideal and more beneficial than looking at the mouse.”
Neuron changes:
The researchers created a collection of brain organoids using skin stem cells from three subjects with a deletion and macrocephaly, three with a duplication and microcephaly, and three controls. They measured the size of the organoids after 6 days, 16 days, 1 month and 3 months in culture.
At all stages of development, deletion-derived organoids tended to be larger than control organoids and duplication-derived organoids smaller, consistent with head size trends in people with the variants.
Sizing: Organoids with 16p11.2 deletions (left) are larger than usual, while those with duplications are smaller (right), mimicking head size trends in people with these variants.
“It’s a very important direct link to the patient’s phenotypes,” says Iakoucheva. “We were very excited when we saw this.”
The results were published in Molecular Psychiatry last week.
The researchers also sequenced RNA from one- and three-month organoids and the stem cells from which they grew. Here, too, the mutated organoids differed in all stages of development from the control organoids in the expression of hundreds of genes; Many of the genes involved facilitate the formation of neurons and their migration into the cortex at an early stage in development. Many also shared functions with other genes strongly linked to autism, such as CHD8.
Both deletion and duplication organoids had elevated levels of an enzyme called RhoA, which helps regulate neuronal migration. When the team put neurons from the organoids in a bowl, those with mutations tended to travel shorter distances than the control neurons. But this difference disappeared in neurons from organoids treated with a RhoA inhibitor.
Careful interpretation:
The organoids can reveal more about open features like total size than about neuron size or synapse density, nuanced features that may be more suitable for 2D models, Song says.
Similarly, he says, RNA sequencing could be more revealing in an animal model, allowing researchers to do single-cell sequencing and understand which cell types are expressing which genes. (Iakoucheva says she hopes to do such sequencing in the organoids in the future.)
“This is really a great start for a big phenotype model,” says Song.
While the work is solid, researchers should be careful not to over-interpret the results, says Verónica Martínez-Cerdeño, professor of pathology at the University of California, Davis. For example, it is unclear whether the differences in organoid size correspond to the differences in head size observed in humans.
“This paper may give us some candidate genes to study in the future, but we just don’t know how it really leads to autism,” says Martínez-Cerdeño.
Still, comparing the effects of deletions and duplications directly is helpful, says Sergiu Pasca, associate professor of psychiatry and behavioral science at Stanford University in California.
“This is particularly important given the contrasting clinical phenotypes in patients,” says Pasca.
Next, Iakoucheva’s team plans to investigate which cell types are disrupted by the changes in RhoA signaling, to analyze the electrophysiological activity of the organoids, and to assess whether the RhoA inhibitor affects the behavior of mice with 16p deletions or duplications.
