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Brain vs. brain
SAN DIEGO—Brain organoids have many uses, among them helping researchers track human development and unraveling the molecular events that lead to disease and test new treatments. They aren’t perfect replicas—brain organoids don’t replicate cognitive function—but researchers can check how an organoid’s physical structure or gene expression changes over time, or as a result of a virus or drug.
In late August, researchers at the University of California, San Diego (UC San Diego), noted that they have taken brain organoids one step further, achieving an unprecedented level of neural network activity. Using data from babies born up to three-and-a-half months premature, the team developed an algorithm to predict their age based upon EEG patterns.
The algorithm read lab-grown brain organoids and assigned them an age. The electrical impulse pattern for nine-month-old brain organoids revealed features similar to those of a premature infant who had reached 40 weeks of age.
The optimized brain organoids, described in the Aug. 29 issue of Cell Stem Cell, could make it possible for researchers to study mental illnesses like autism or epilepsy that aren’t caused by or result in overt physiological changes, but instead involve disturbances in brain cell network activity. For many of these conditions, there are no relevant laboratory or animal models.
“We couldn’t believe it at first; we thought our electrodes were malfunctioning. Because the data were so striking, I think many people were kind of skeptical about it, and understandably so,” said co-senior author Dr. Alysson R. Muotri, professor of pediatrics and cellular and molecular medicine at UC San Diego School of Medicine, director of the UC San Diego Stem Cell Program and a member of the Sanford Consortium for Regenerative Medicine. Muotri led the study with Dr. Bradley Voytek, associate professor of cognitive science in the UC San Diego Division of Social Sciences.
As Muotri tells DDNews, “Advanced network for computing is what the brain does. Having brain organoids with only morphology preserved was not enough to study neurological conditions, such as autism or epilepsy. We can make brain organoids from autistic individuals, for example. Then, we compare the networks from them to organoids derived from neurotypical individuals.”
At UC San Diego brain organoids have been used to produce proof that the Brazilian Zika virus can cause severe birth defects, and to repurpose existing HIV drugs for a rare, inherited neurological disorder. Muotri and team also recently sent their brain organoids to the International Space Station to test microgravity’s effect on brain development.
In the latest study, Muotri and colleagues optimized every step of brain organoid construction. They started from single cells, rather than the clumps of cells used in most protocols. The researchers also tweaked the precise timing and concentration of factors added to prompt brain cell organization.
“We improved many steps of the process—for example, dose optimization and exposure time to factors that are required for the neutralization of the pluripotent stem cells. We also optimized the conditions to keep these organoids in a more physiological formula so neurons could survive longer and make functional synapses,” Muotri notes.
The optimization paid off in terms of cellular diversity and cellular network activity. The team detected a cortical GABAergic neuron that had never before been generated in a lab dish—an important player in the sophistication of neural networks.
“We actually showed that both glutamatergic and GABAergic neurons are essential for the brain oscillations to appear,” he added. “Not only do you need them, but you also need them to appear in a synchronized fashion.”
The researchers grew their newly optimized brain organoids on multi-electrode arrays, to measure cellular network activity. The electrodes capture and record electrical impulses, which appear as patterns of waves and spikes in an EEG read-out. With the new protocol, the brain organoids went from producing 3,000 spikes per minute to 300,000 spikes per minute.
“[We] don’t need an entire human brain in uterus, connected with all other tissues to the body, to form the networks. A small version of the human cortex, such as in our brain organoids, can generate oscillatory waves in the absence of inputs,” states Muotri.
In humans, oscillations change with age as brain cell connectivity develops. Newborn baby brains tend to have periods of rest (no waves) between spikes of electrical activity. Those quiet periods get shorter and shorter as the brain develops. In time, brain activity becomes constant, though levels vary. These brain oscillation patterns often correlate with human cognition and disease states.
Muotri and his team compared their brain organoid electrical patterns to a publicly available dataset of 567 EEG recordings from 39 babies born prematurely (between 24 and 38 weeks gestation) and for several weeks after birth. From their initial days to nine months, the brain organoids produced similar levels of electric activity, following a similar pattern: less quiet time and more frequent electrical impulses.
“They are far from being functionally equivalent to a full cortex, even in a baby,” said Muotri. “In fact, we don’t yet have a way to even measure consciousness or sentience.”
Muotri’s brain organoids can live for years in the lab, but their activity plateaus at nine months. A number of reasons might apply, including lack of blood vessels or the need for additional neurons to continue maturing.
“To me, the most fascinating aspect of this work is that the cells have encoded all the information to assemble such sophisticate networks in their genomes,” he points out. “The brain is genetically pre-programmed to generate sophisticated networks even outside the human body.”
The more brain organoids can replicate human brains in the lab, the less researchers will have to rely on animal models and fetal tissue to understand and treat human disease.
“Our work doesn’t yet replace the need for human fetal brain tissue for research, but it’s very attractive as a potential alternative,” Muotri concludes.