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New understanding of myelin-producing brain cells could prove useful in studying demyelination disorders

Researchers at Stanford University School of Medicine have proved that a system they developed a few years ago for culturing balls of stem-cell-derived human brain cells, which mimic aspects of real brain circuitry, can generate oligodendrocytes together with neurons and a third type of brain cell called astrocytes.

A better understanding of oligodendrocytes, the brain cells that make myelin, might help correct or prevent conditions such as multiple sclerosis (MS), but studying them has been tough. They’re born late in brain development, and they’re challenging to generate alongside human neurons and other brain cells in a way that recapitulates the complex interactions occurring among these cell types as they develop.

“We now have multiple cell types interacting in one single culture,” said Sergiu Pasca, MD, assistant professor of psychiatry and behavioural sciences. “This permits us to look close-up at how the main cellular players in the human brain are talking to each other.”

A study describing the work was published online in the journal Nature Neuroscience.

Three cell types developed from pluripotent stem cells in a dish. They arranged themselves within three-dimensional ‘brain balls’, or spheroids. They progressively advance in maturity and engaged in lifelike interactions with one another. The scientists observed the oligodendrocytes’ movements and watched them wrap their extensions around individual neurons to form the insulating coats of myelin that, in real-life brain tissue, speed up signal transmission.

As a result, the researchers were able to determine which genes were active at different stages of oligodendrocyte development in the brain spheroids, and to show that these gene-activation patterns were extremely similar to those of real-life oligodendrocytes at comparable stages of maturation. This enabled them to pinpoint, in these cultured oligodendrocytes, the different times of onset or activation of several genes that, when mutated, cause different congenital myelination disorders — a finding with possible implications for modelling these disorders.

These spheroids contained as many as 1 million cells and measure as much as one-eighth of an inch in diameter. They survived in culture for at least two years. During that time researchers witnessed another type of brain cell, called astrocytes, manifest. These cells outnumber neurons in the brain and perform many essential tasks, from managing nutrition and energy supplies for neurons to directing the positioning, formation and functionality of synapses, the junctions through which neurons transfer information to one another.

In the human cerebral cortex, most neurons are born by week 26 of gestation. Astrocytes begin to appear around this period and continue to mature for months afterward. Previous culture methods didn’t keep cells alive long enough to recapitulate this maturation process.

Human oligodendrocytes take even longer to make their appearance. In higher brain regions such as the cerebral cortex, responsible for advanced cognitive functions such as decision-making, scheduling and foresight, oligodendrocytes begin to form in significant numbers around the time of birth.

In the new study, the researchers modified their previous method of culturing brain spheroids by adding special growth factors and nutrients that promote oligodendrocyte formation, survival and development. By day 100 of culture initiation, oligodendrocytes were present alongside neurons and astrocytes.

Using live-imaging microscopy, they were able to see into brain spheroids and record the behaviour of oligodendrocytes that had been labelled with a fluorescent marker. This was extensively observed between days 65 and 275, monitoring different cells’ behaviour by varying the microscope’s depth of field. The researchers watched oligodendrocytes migrating from their points of origin to their neuronal destinations.

High-resolution electron microscopy revealed oligodendrocyte extensions sheathing neuronal filaments within three to four months of culture initiation.

When the scientists exposed the brain spheroids to a fat-dissolving emulsifier, “oligodendrocytes were affected the most,” Pasca said. “It was as though they were melting.”

Generating brain spheroids from patient-derived skin cells allows medical researchers such as Pasca to study neurological and psychiatric diseases on a personalised basis without having to obtain and maintain living brain tissue.

Pasca’s group is looking at a number of genetic disorders affecting myelination that arise in foetal development or early childhood. But oligodendrocyte-containing brain spheroids could also prove useful in studying demyelination disorders, such as MS.

Source: MS-UK, 31/01/2019

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