By using stem cells to grow miniature brain-like organs in the lab, scientists have opened up new avenues for research into neurodevelopment, diseases and therapies that cannot be performed in living humans. But not all mini-brain organoids are created equal, and getting them to exactly mimic the human brain tissues they model has been a persistent challenge.
“Right now it’s like the Wild West because there’s no standard method for generating mini-brain organoids,” said Bennett Novitch, a member of the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA and the senior author of a new article on the subject. “Every neuroscientist wants to create a brain organoid model of their favorite disease, and yet everyone’s organoids don’t always look alike.”
Because there is no common protocol for their production and because there are no quality control guidelines, organoids can vary from lab to lab — and even batch to batch — meaning a finding in one organoid may not be true in another.
“If my lab and another lab down the hall performed drug screening using mini-brain organoid models of the same condition, we could still get different results,” said Momoko Watanabe, the new paper’s lead author and an assistant. professor of anatomy and neurobiology at UC Irvine. “We won’t know whose findings are correct because the differences we see may be reflections of how our models differ rather than reflections of the disease.”
In their new study, published today in Stem Cell ReportsNovitch, Watanabe and their colleagues, based on their research, propose guidelines that could help scientists overcome two major obstacles to the full potential of these organoids: differences in uniformity and structure.
Having organoids that accurately and consistently mimic the structure and cellular makeup of specific areas of the brain is especially important for studying conditions such as schizophrenia and autism spectrum disorder where the brains of affected people often appear identical to neurotypical brains in structure, yet show clear differences in function.
“We’ll never be able to identify the subtle differences in brain structure and function — things that are relevant to patients with neurological disorders — if our organoids have the wrong balance of cell types or grossly irregular structure,” said Novitch, who is also director of the Integrated Center for Neural Repair at the UCLA Brain Research Institute.
Making the best organoids: a matter of maturity
To produce mini-brain organoids, which can be 1 to 5 millimeters in diameter, scientists first take human skin or blood cells and reprogram them to become induced pluripotent stem cells — cells that can differentiate into any cell type in the body. They then direct these iPS cells to make neural stem cells, which can produce most cell types in the brain. As the neural stem cells form, they can be coaxed to aggregate into 3D organoids. Simple enough. But why do some organoids resemble the human brain better than others?
To answer this question, the team collaborated with pluripotency experts Kathrin Plath and Amander Clark of the UCLA Broad Stem Cell Research Center. They found that the developmental maturity of the stem cells from which an organoid is grown influences the quality, just as the freshness of ingredients influences the quality of a culinary dish.
“In human embryonic development, the nervous system is one of the first structures to form, so it makes sense that stem cells early in development are best at producing brain organoids,” said Watanabe, who is also a member of the UCI Sue. & Bill Gross Stem Cell Research Center.
The researchers then found that the best way to keep human stem cells in an early developmental state suitable for organoid formation was to grow them in a dish of mouse skin cells called fibroblast feeders, because they provide essential chemical signals and structural support. which helps stem cells expand and maintain their immaturity over time.
Unfortunately, they also found that using mouse cells could make organoids less suitable for developing cellular therapies to replace diseased or damaged neural tissues. Furthermore, these feeder-assisted methods are more cumbersome than the stem cell growth methods commonly used by many labs.
The team then turned to RNA sequencing and computational analysis in an attempt to pinpoint genetic differences between stem cells that produce good organoids and those that don’t. This allowed them to identify four molecules — all belonging to the transforming growth factor beta superfamily of molecules — that were responsible for keeping stem cells in a less developed state.
Adding these four molecules to stem cells growing in a dish kept them immature and allowed these cells to produce high-quality, well-structured organoids.
“We found a way to have our cake and eat it too,” Novitch said. “We took mouse cells out of the equation while retaining some of their organoid formation benefits, bringing us closer to our goals of studying and developing treatments for complex neurological diseases.”
Momoko Watanabe et al, TGFβ superfamily signaling regulates the state of human stem cell pluripotency and the ability to create well-structured telencephalic organoids, Stem Cell Reports (2022). DOI: 10.116/j.stemcr.2022.08.013
Quote: Making lab-grown brain organoids ‘brainier’ (2022, September 29) retrieved September 29, 2022 from https://phys.org/news/2022-09-lab-grown-brain-organoids-brainier.html
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