In research published in the journal Cell Stem Cell--in a study entitled, Induction of expansion and folding in human cerebral organoids--researchers provide insight into a specific gene pathway that appears to regulate the growth, structure, and organization of the human cortex. They also demonstrate that 3D human cerebral organoids--miniature, lab-grown versions of specific brain structures--can be effective in modeling the molecular, cellular, and anatomical processes of human brain development. And they suggest a new path for identifying the cells affected by Zika virus.

With normal neural progenitor cells (NPs), the human organoid developed into relatively small cell clusters with smooth surface appearance, displaying some features of very early development of a human cortex. However, deleting PTEN allowed the progenitor population to continue expanding and delayed their differentiation into specific kinds of neurons--both key features of the developing human cortex. "Because the PTEN mutant NPs experienced more rounds of division and retained their progenitor state for an extended period, the organoids grew significantly larger and had substantially folded cortical tissue," explains a lead author.

The Whitehead investigators chose to focus on the PTEN gene because it had previously been shown to have some function in cortical development and to have a role in regulating progenitor cells of various lineages. Notably PTEN loss-of-function mutations have been associated with human macrocephaly.

In this study, deletion of the PTEN gene increased activation of the PI3K-AKT pathway and thereby enhanced AKT activity in the human NPs comprising the 3D human cerebral organoids; it promoted cell cycle re-entry and transiently delayed neuronal differentiation, resulting in a marked expansion of the radial glia and intermediate progenitor population. Validating the molecular mechanism at work with PTEN, the investigators used pharmacological AKT inhibitors to reverse the effect of the PTEN deletion. They also found that they could regulate the degree of expansion and folding by tuning the strength of AKT signaling--with reduced signaling resulting in smaller and smooth organoids, and increased signaling producing larger and more folded organoids.

Finally, the researchers utilized the 3D human cerebral organoid system to show that infection with Zika virus impairs cortical growth and folding. In the organoids, Zika infection at the onset of surface folding (day 19 of development) led to widespread apoptosis; and, ten days later, it had severely hampered organoid growth and surface folding. Zika infection of 4-week-old organoids, showed that PTEN mutant organoids were much more susceptible to infection than normal control organoids; notably, they showed increased apoptosis and decreased proliferation of progenitor cells.