Neurology Central

Brain organoids: defining maturity, addressing the challenges and exploring future directions

Alysson Muotri is a Professor at the School of Medicine at the University of California in San Diego (CA, USA). His research focuses on modeling neurological diseases, such as autism spectrum disorders, using induced pluripotent stem cells. Alysson’s lab has developed several techniques to culture functional brain organoids for basic research and drug-screening platforms.
In this interview, Alysson speaks to us about his work with organoids to study the mechanisms of brain development. He also discusses the challenges he faces when using organoids in his work, such as automation, optimization and vascularization, and how we might address these obstacles.

1.Your lab group has done an impressive amount of work using organoids to study the mechanisms of brain development. Could you tell us more about this and why you began to use organoids in your research?

My lab has been interested in human brain development and evolution for the past 10 years. Over time, we have perfected and optimized our methods to create a dynamic protocol to generate functional brain organoids using a simple and robust recipe. These brain organoids recapitulate early stages of embryonic/fetal human development that is implicated in several neurodevelopmental conditions, such as autism.

The inaccessibility of the human brain in utero makes this model very relevant and orthogonal with other models, such as post-mortem tissues and animal models. Nonetheless, it is important to keep in mind that the brain organoid models also have inherent limitations: we don’t have a representation of all cell types, there is no vascularization, our culture conditions are not exactly like the human uterus, etc., to list a few. Thus, it is important that observations made on this highly artificial in vitro system be validated using primary tissues or even live brains.

2.One question surrounding the use of organoids for neurodevelopmental disorders is whether the cells in the culture develop at the same rate as human cells. How do we define the cell’s maturity in these systems?

Great question. In the past, the only way to do this comparison was to match global gene expression from the brain organoids with post-mortem tissues. However, the global gene expression coming from dead tissue suffers from RNA degradation as often the tissue is not well-preserved. We are using an alternative, which is to compare the maturity of networks using multielectrode arrays (MEA) directly to the intact live brain. Using this electrophysiological method, we find that, surprisingly, the maturation of the neuronal networks on the brain organoids are quite similar to normal human neurodevelopment, starting at 25 weeks post-conception to birth (9-month-old organoids).

3.In your opinion, do you think we are close to having a standard definition of ‘mature’ for organoids? If not, what steps are required for us to get there?

Yes, I am optimistic. I think both gene expression and electrophysiology are great parameters to measure maturity and many people would agree with this standard.

4.What are some of the key challenges you face when using organoids in your work?

These are highly hands-on protocols. So far, we don’t have good automation that could substitute the human eye. Thus, experimental reproducibility is a challenge. Also, we would like to mature these organoids for years and the methods need to adapt to long-term cultures. Finally, I think a challenge for the whole field is to create functional circuits in vitro.

5.How could these challenges be addressed?

Automation is inevitable. We will need automation for high-throughput screenings and disease modeling. I think that bringing bioengineers to this field will help. This should also help us with culture condition optimization and even vascularization (bioprinting endothelial cells is a strategy we are now pursuing). Once implemented, these technical advances will generate more robust and reproducible protocols. The endogenous vascularization should help keep long-term cultures and, as a consequence, maturation. To create circuits, we will need to understand more biology and patterning factors that could guide the stem cells into specific fates and brain regions. The bioengineering will be necessary again to create substrates for these neurons to connect.

6.Lastly, where do you think we could be 10 years from now and where might the best use for organoids be?

I think in 10 years we would have solved most of the technical limitations and our understanding of the biology will improve, helping us to create a diversity of regional-specific brain organoids. In 10 years or less we will be collecting the fruits of this technology by recreating circuits in a lego-organoid fashion and applying it to disease modeling. That will allow us to get closer to manipulate specialized circuits with precision, a key concept for a more personalized medicine.

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The opinions expressed in this interview are those of the interviewee and do not necessarily reflect the views of Neuro Central or Future Science Group.