A new study, published in Cell Stem Cell, has grown brain organoids in the lab that developed basic eye structures, which are capable of sensing light. The generated models have the potential to influence future treatment of early retinal diseases.
Brain organoids
The complex nature of the human brain it very difficult to study. To help develop our understanding of this organ, brain organoids have been widely used. Brain organoids are three-dimensional brain models derived from induced pluripotent stem cells (iPSCs). They can be used to study brain development, diseases, the effect of drugs and more.
Eye development is a complicated process that is linked to the brain. The process begins during embryogenesis when optic vesicles develop from the forebrain. Whilst retinal organoids have previously been created in the lab, these do not provide an accurate model of optic vesicles. This is likely due to the fact that retinal organoids do not contain any brain tissue. Therefore, creating a model where eye development can be studied in detail could allow for the identification of molecular mechanisms that underlie early retinal diseases, and facilitate the development of improved treatments.
This study overcame this problem by developing, for the first time, brain organoids complete with optic cups. Optic cups are structures that comprise a portion of the optical vesical. They are found in the eye where the optic nerve meets the retina. Amazingly, the organoids developed in this study were generated within 60 days, a time frame that seemingly matches that of human embryonic retina development.
Encouraging differentiation into optic cells
To encourage the development of optic cups, the team used retinol acetate to induce iPSCs to differentiate into optic cells. Next, single-cell RNA sequencing (scRNA-seq) was used to investigate cell diversity in the organoids at 30-days old, to see whether the iPSCs were differentiating. Encouragingly, the researchers found clusters of developing optic vesicles and radial glial which were segregated from neuronal cell types.
The continuous culturing of their organoids resulted in the progressive development of the optic vesicle clusters. Organoids which successfully developed optic vesicles that attached to the forebrain were termed optic vesicle brain organoids (OVB-organoids). Overall, 66% of organoids in the study transformed into OVB-organoids. Interestingly, in all OVB-organoids, the optic vesicles formed at one pole of the organoid. This suggests that a specific area in the forebrain is responsible for optic development.
Functional ‘eyes’
Crucially, the optic cups developed by the organoids in this study were functional. Along with retinal cell types, they amazingly also contained lens and cornea tissue, which are formed from non-neuronal cell types. In addition, the neural retina in OVB-organoids was seen to have a well-developed retinal pigment epithelium (RPE) that was connected to the forebrain. RPE has several key functions, including light absorption.
The diverse range of developed retinal cells formed neuronal networks that were actually able to respond to light and send corresponding signals into the brain. The photoreceptors were also found to recover their light sensitivity after photobleaching with intense flashlight stimulation. The team repeated the photobleaching experiment using mice lacking RPE. The mice did not recover light sensitivity, highlighting the importance of RPE and functional integration with the brain.
“In the mammalian brain, nerve fibres of retinal ganglion cells reach out to connect with their brain targets, an aspect that has never before been shown in an in vitro system,” Jay Gopalakrishnan, senior author of the study, said.
Conclusion
The brain organoids generated in this study displayed remarkable development of a diverse range of optical cells. This allowed for a novel model of early eye development. However, the viability of the OVB-organoids beyond 60 days is questionable. Consequently, the OVB-organoids in this study could not develop mature retinal cell types. Further studies will be needed to investigate strategies that will allow organoids to be cultured for longer.
Despite this, the team is optimistic that their OVB-organoids will greatly advance understanding of both human brains and eye development. This will pave the way for future disease treatment.
“Our work highlights the remarkable ability of brain organoids to generate primitive sensory structures that are light sensitive and harbour cell types similar to those found in the body,” said Gopalakrishnan. “These organoids can help to study brain-eye interactions during embryo development, model congenital retinal disorders, and generate patient-specific retinal cell types for personalized drug testing and transplantation therapies.”
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