‘Mini-brains’, or lab-grown clumps of neurons, are a groundbreaking new technology that scientists are using to learn more about how our brain works.

“What makes the human brain unique?”

Dr. Madeline Lancaster, a neurobiologist at the University of Cambridge,  has focused her entire career on answering this one question [2]. After millions of years of evolution, humans remain the only species capable of complex thought and consciousness. So what makes our brains so different from other animals?

It’s easy to see why this is such a fascinating topic. Each of our brains contains over 100 billion neurons, all compacted into a space smaller than two of your fists. The brain also has over 100 trillion connections, around the same number as stars in the biggest galaxies. In a way, our brains can be likened to the connections in a supercomputer, but infinitely more complex. These connections allow us to invent airplanes, paint Van Gogh’s Starry Night, and feel the full range of emotions that are a part of the human experience, allowing us to think, cry, hate, and love. This makes our brain extremely interesting but also challenging to study directly. 

Instead, scientists have to scale down and find simpler, but still accurate, models of our brain. At the time, the most widely-used models were mouse models, brain scans, and postmortem brain slices. However, these models fall short in providing a brain replicate that can visualize living, human neurons. Therefore, short of probing an actual human developing brain, Lancaster needed to create a better model to study the development of the human brain [3]. 

To aid in this endeavor, she recruited the help of Dr. Hans Clevers, a stem cell scientist from the Netherlands. Clevers pioneered the use of stem cells to generate other ‘mini organs’ such as intestines, lungs, and the pancreas. He found that stem cells can be directed to turn into any cell type with the right conditions and nutrients. When these cells are cultured in liquid suspension, they can spontaneously arrange themselves into a 3D configuration and self-organize to reflect how the cells are structured in the actual organ. 

Together, with the knowledge of their respective fields, Lancaster and Clevers aimed to differentiate stem cells into neurons and effectively build a brain from stem cells. They spent years nailing down the nutrients required and when to add them, drawing from recent research to assess which molecules are necessary for a developing brain at what time.   

The idea of scientists growing mini-brains in a lab sounds like something straight out of The Matrix, but in reality, the technology has been around for a decade. In 2013, after years of effort, Lancaster was finally able to develop the first ‘mini-brain organs’ using stem cells [4]. The more technical term for these ‘mini-brains’ sounds a little less sci-fi-esque. Brain organoids are lentil-sized clumps of neurons that are able to spontaneously arrange themselves in a sphere-like configuration and function in a way that models the human brain. On the surface, they look like nothing more than beige spheres just a few millimeters across, but the inside holds so much more. In these organoids, you can find most of the neuronal cell types in your brain arranged into their respective layers representing the retina, spinal cord, and memory regions (hippocampus) [5]. Scientists have also found that the neurons in brain organoids can form up to 80 million connections with other neurons and have similar electrical activity as a fetus in early brain development – all in a sphere less than 4mm in diameter. 

Can brain organoids be used to study brain development? 

A core tenet of modern neurobiology is that the human brain is the most complex and well-connected organ in the animal kingdom. So what makes human brains so unique? How do humans, unlike other animals, develop consciousness and learn complex languages?

According to scientists like Lancaster, the answer to this elusive question lies in the development of the human brain. Understanding brain development will give scientists insight into how our neurons form cell-to-cell connections, why we have so many more connections than other organisms, and how different neural circuits function to help us think, feel, and speak. Organoids offer remarkably accurate insights into the first ten weeks of brain development in a human fetus as they contain the same proteins, cell types, electrical activity, and cell layers across the first ten weeks of development as a fetal brain before the first trimester [6]. While it’s almost impossible to study fetal brain tissue ethically, brain organoids are an accessible and easily manipulable model to use instead. 

One central question that brain organoids have helped to answer is the mechanism behind how the brain folds in on itself. All together, the unfolded surface area of the brain adds up to the approximate surface area of your pillowcase, which clearly could not fit into our heads. However, the brain folds itself in a way that maximizes the surface area of the brain while minimizing the total volume of space that it takes up. The mechanism behind this largely remained a mystery until organoids came along. 

Seeking to answer this question, Dr. Qinying Wang, a neurobiologist at the Chinese Academy of Sciences, focused her attention on a protein expressed at very high levels during development. This protein, DRD1, is specifically expressed on the outer surface of the fetal brain. Wang then used CRISPR, a gene editing tool, to develop brain organoids that overexpressed the DRD1 gene [7]. The results were immediately noticeable. Within a few hours, the organoids exhibited extensive folding and wrinkling throughout. This suggested that the protein has an essential role in brain folding, which has vast implications for those suffering from brain folding disorders such as lissencephaly. This discovery also led to the development of the drug Risperidone, which is used to treat Schizophrenia by targeting a closely related protein: DRD2.   

Can organoids be used as a model for neurodegenerative disease? 

Neurodegenerative disorders such as dementia and Alzheimer’s disease are devastating diseases that affect millions of people every year. Scientists have developed organoid models for these diseases using patient-specific stem cells [9]. These diseased organoid models have shown how neuron-to-neuron dynamics change in response to specific disorders. Scientists can now test potential drugs on these organoids before moving on to clinical trials. In this way, brain organoids have revolutionized the pipeline between the lab bench and applied therapeutics. 

Dr. Jing Zhao, a researcher at the Mayo Clinic, has pioneered the use of brain organoids to study Alzheimer’s Disease. Zhao knew that the mutated APOE4 gene is one of the strongest risk factors for developing Alzheimer’s Disease. Scientists have found that the mutated APOE4 gene increases cell death, but the mechanisms behind this are a mystery. Using organoid technology, Zhao examined the synaptic connections arising from organoids with the mutated gene and found that neurons were unlikely to form synapses with other neurons and were prone to accumulate cell waste [10]. This has led to clinical applications that use gene therapy to introduce unmutated versions of the APOE4 gene into patients. 

The future of brain organoids 

The future of brain organoids is bright – not just for scientists, but for all of us. The human brain is the most advanced organ we know of, and any model that is even a fraction as complex is immensely valuable to our understanding of how our brains tick. 

According to Dr. Lancaster, brain organoids may become invaluable to gene therapies [11]. Organoids constructed from patient-specific stem cells could open up the field of individualized gene and drug therapy. A significant issue with drug therapy is the difficulty in predicting how a particular drug will affect different people. Individualized drug therapy is an emerging field that takes a holistic view of patients – taking into account each patient’s genetic and epigenetic history- to personalize their medication. This would eliminate any risk of potentially life-threatening drug side effects. Patients and their families can then make more well-informed decisions about their medications. Beyond drug development, individualized brain organoids can also be used in genome editing alongside genetic and epigenetic screening to help patients make informed choices about their health. This would revolutionize the field of personalized medicine and prenatal diagnostics. 

References

[1] Lancaster Lab, https://www2.mrc-lmb.cam.ac.uk/groups/lancaster/.

[2] IEEE Pulse. “Steering Organoids toward Discovery.” IEEE Pulse, IEEE Pulse //Www.embs.org/Pulse/Wp-Content/Uploads/Sites/13/2022/06/Ieee-Pulse-logo2x.Png, 4 Mar. 2022, https://www.embs.org/pulse/articles/steering-organoids-toward-discovery/.

[3] Knoblich, Juergen A. “Building a Brain in the Lab.” Scientific American, Scientific American, 1 Jan. 2017, https://www.scientificamerican.com/article/building-a-brain-in-the-lab/.

[4] Lancaster, Madeline A., et al. “Cerebral Organoids Model Human Brain Development and Microcephaly.” Nature News, Nature Publishing Group, 28 Aug. 2013, https://www.nature.com/articles/nature12517.

[5] Lee, Chun-Ting, et al. “3D Brain Organoids Derived from Pluripotent Stem Cells: Promising Experimental Models for Brain Development and Neurodegenerative Disorders – Journal of Biomedical Science.” BioMed Central, BioMed Central, 20 Aug. 2017, https://jbiomedsci.biomedcentral.com/articles/10.1186/s12929-017-0362-8.

[6] Cleber A. Trujillo 1, et al. “Brain Organoids and the Study of Neurodevelopment.” Trends in Molecular Medicine, Elsevier Current Trends, 28 Oct. 2018, https://www.sciencedirect.com/science/article/pii/S1471491418301862?via%3Dihub.

[7] Lancaster, M., Renner, M., Martin, CA. et al. Cerebral organoids model human brain development and microcephaly. Nature 501, 373–379 (2013). https://doi.org/10.1038/nature12517

[8] “Organoid.” Attisano Lab & Wrana Lab, http://attisanowranalabs.science/organoid.

[9] Zhang DY, Song H, Ming GL. Modeling neurological disorders using brain organoids. Semin Cell Dev Biol. 2021 Mar;111:4-14. doi: 10.1016/j.semcdb.2020.05.026. Epub 2020 Jun 17. PMID: 32561297; PMCID: PMC7738381.

[10] Zhao, J., Fu, Y., Yamazaki, Y. et al. APOE4 exacerbates synapse loss and neurodegeneration in Alzheimer’s disease patient iPSC-derived cerebral organoids. Nat Commun 11, 5540 (2020). https://doi.org/10.1038/s41467-020-19264-0

[11] Iva Kelava, et al. “Dishing out Mini-Brains: Current Progress and Future Prospects in Brain Organoid Research.” Developmental Biology, Academic Press, 9 July 2016, https://www.sciencedirect.com/science/article/pii/S0012160616302196?via%3Dihub.