The entire tissue shapes brain development

In humans, the neocortex is the seat of cognition. It enables us to think, imagine, and dream. During embryonic development, a kaleidoscope of neurons and glial cells—non-neuronal cells in the brain—arises from a large pool of stem cells. This complex process, however, is prone to errors: mistakes can lead to the formation of too few or too many neurons, resulting in conditions such as microcephaly or macrocephaly.

Color-coded neurons are key

Simon Hippenmeyer and his group at the Institute of Science and Technology Austria (ISTA) sought to understand the mechanisms that control the generation of stem cells, neurons, and glia. They investigated these mechanisms both at the level of individual stem cells and within the holistic context of the entire tissue. To do this, they used the MADM technique established by Hippenmeyer. With MADM, scientists can color-code cells in the brain and simultaneously switch off specific genes involved in brain development, either in individual cells or globally across the entire brain. "This allows us to visualize individual cells and see how they react when specific genes are switched off, either in the entire tissue or just in a single cell," explains Nicole Amberg, a former postdoctoral researcher in Simon Hippenmeyer's group.

The neuroscientists paid particular attention to the role of PRC2. PRC2 is a so-called epigenetic regulator, meaning that it controls the degree of DNA packaging. The loss of this protein complex leads to dramatic microcephaly in mice: their neocortex is only half the size of that of genetically unaltered mice. How PRC2 regulates the production of neurons was previously unknown. However, since epigenetic complexes usually act on the DNA in the cell nucleus, it was concluded that PRC2 most likely only acts within each individual stem cell. Hippenmeyer and Amberg decided to test this idea. “With MADM, we can track whether stem cells produce the right types of neurons in the right numbers, both when PRC2 is lost in a single stem cell and when it is lost in the entire tissue—we then compare these results,” says Amberg.

When the researchers removed PRC2 from a single stem cell, the mice's neocortex remained unchanged. “The stem cell doesn't really care and produces a normal number of neurons. When PRC2 is removed from a single stem cell, the neuronal part of the neocortex doesn't change at all: We see the same number of neurons, the same subtypes, in the correct proportions,” Amberg recalls. The observation puzzled them because switching off PRC2 in the entire tissue results in only half the number of neurons being formed. “So where is the problem? These results point to a mechanism that operates at the level of the entire tissue.”

Neighboring Cells Influence Stem Cell Development

The researchers set out to investigate and examine gene activity. They found dramatic differences in which genes were active and which were not, depending on whether they knocked out PRC2 in a single stem cell or in the entire tissue. When they knocked out PRC2 in the entire developing neocortex, the gene was also deregulated in neighboring cells, including the stem cells. This suggests that the loss of PRC2 in the stem cell niche influences stem cell development. “We didn’t expect PRC2 to play such a far-reaching role. It’s surprising that neighboring cells are so important for the stem cell—especially since PRC2 doesn’t code for a receptor or signal, but acts within the cell nucleus,” says Hippenmeyer.

How the loss of PRC2 in the entire tissue mechanistically affects stem cell development is not yet clear. Further results showed that the loss of PRC2 leads to waves of gene deregulation. “Switching off the PRC2 complex alters gene expression, which then triggers a second wave of deregulation. And the genes in this second wave are crucial for the microcephaly phenotype,” explains Amberg.

While the loss of PRC2 in a single stem cell does not impair neuron development, this loss does affect the development of astrocytes—an important type of glial cell: After a stem cell has produced precursor cells for neurons, it normally produces precursor cells for astrocytes. However, if PRC2 is lost in the stem cell, these astrocytes do not develop properly.

Although the study was conducted on mice, the findings are also relevant to human development, says Hippenmeyer. “A global gene knockout, meaning the deactivation of a gene across the entire tissue, can indeed have serious consequences. Furthermore, an inherited mutation affecting the entire tissue can lead to a more severe outcome than a mutation acquired later in embryonic development, when only a few cells are affected.”

Publication:

Amberg et al. 2022. Tissue-wide Genetic and Cellular Landscape Shapes the Execution of Sequential PRC2 Functions in Neural Stem Cell Lineage Progression. Science Advances.

DOI: 10.1126/sciadv.abq1263

http://www.science.org/doi/10.1126/sciadv.abq1263

Project Funding:

The first author received funding through the FWF Firnberg Program (T 1031). This work was supported by ISTA institutional funds and by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No. 725780 LinPro) for Simon Hippenmeyer.