Stem cells tweaked in the laboratory have allowed researchers, reportedly for a first time, to generate and maintain ball-shaped cultures — called spheroids — of human brain cells in 3D that contain oligodendrocytes, the cells that produce myelin, along with neurons and the astrocytes that are essential to nerve cell health.
These long-surviving spheroids (which the researchers call “human oligodendrocyte spheroids”) will help scientists in studying how oligodendrocytes develop and interact with other brain cells, and what happens when they lose the ability to regenerate myelin — the protective coating on nerve cell fibers that promote cell-to-cell communication. Myelin’s loss, called demyelination, marks diseases like multiple sclerosis (MS).
The study “Differentiation and maturation of oligodendrocytes in human three-dimensional neural cultures” was published in the journal Nature Neuroscience.
Understanding how oligodendrocytes work and interact with other brain cells, such as neurons or astrocytes, is vital to developing therapies that might halt or even prevent diseases like MS. Of note, astrocytes are a group of star-shaped cells that provide neurons with energy, and work as a platform to clean up their waste; other functions they perform within the brain include regulating blood flow and inflammation.
While human nerve cells revealed many of their secrets years ago, those of human oligodendrocytes are still far from known. Not only do they appear late in brain development, but they’ve also been much more challenging to study in the lab (in vitro). And, as the study noted, there is a “limited accessibility of functional human brain tissue.”
Researchers at Stanford University School of Medicine developed a way to use human induced pluripotent stem cells (hiPSC) — which are able to generate almost any type of cell in the body — and direct them to mature (differentiate) into human neurons, astrocytes, and eventually into oligodendrocytes.
“We now have multiple cell types interacting in one single culture,” Sergiu Pasca, MD, an assistant professor of psychiatry and behavioral sciences and the study’s lead author, said in a Stanford news release written by Bruce Goldman.
The new system, Pasca added, “permits us to look close-up at how the main cellular players in the human brain are talking to each other.”
By week 26 of in-utero brain development of a baby, most neurons or nerve cells and astrocytes exist, and continue to mature in the following months.
Oligodendrocytes begin to appear much later. Those in the cerebral cortex, a brain region responsible for complex cognitive functions such as decision-making, scheduling, and foresight, begin populating the brain around the time of birth.
These three cell types — oligodendrocytes, neurons, and astrocytes — grown in the 3D brain spheroids, maintained the same architecture and interactions as those seen in the human brain.
Spheroid oligodendrocytes in the lab also transitioned through the same developmental steps as oligodendrocytes in the human brain, determined by analyzing their gene expression patterns. “[T]heir morphology changed as they matured over time in vitro and started myelinating neurons,” the study reported.
The researchers could also pinpoint at which stage of oligodendrocyte development certain genes, when mutated to cause various inherited myelination disorders, were activated.
“These findings suggest that having access to multiple stages of oligodendrocyte development may be important for disease modelling,” they wrote.
The brain spheroids developed, containing up to 1 million cells, and survived in culture for at least two years — an important advance over previous cell culture work that saw cells dying before they could mature. Stanford researchers added special growth factors and nutrients to the media in which the brain organoids grow to induce oligodendrocyte formation and long-term survival. These key myelin-generating cells appeared, alongside neurons and astrocytes, within 100 days of culture.
“[W]e anticipate that this method can be used to study oligodendrocyte development, myelination, and interactions with other major cell types in the CNS [central nervous system],” the researchers wrote.
Supporting their potential for disease modeling, the team also treated the spheroids with lysolecithin, a compound known to cause demyelinationin living organisms (in vivo). The treatment induced the loss of oligodendrocyte cells, a process monitored by imaging the cells live.
“[O]ligodendrocytes were affected the most,” Pasca said. “It was as though they were melting.”
In the future, human oligodendrocytes “may also potentially be combined with autologous patient-derived immune cells to study neuro-immune dysfunction, such as in multiple sclerosis,” the researchers wrote.
An ability to scale-up and increase the number of brain spheroids also implies that this system may one day help in identifying molecules able to modulate myelination.