Brain cancer is a fatal disease affecting hundreds of thousands of people around the world. In 2019, about 246,000 people died from brain and central nervous system cancers (Fan et al., 2022). The condition is the most prevalent form of solid cancer in the United States, affecting a variety of individuals, particularly children. Developing novel immunotherapy strategies that can be tailored to patients’ specific needs has shown promise in recent years. However, current brain tumor research relies heavily on mouse models or cultured human tumor cells, which do not fully capture the complexity of the tumor microenvironment. These models have several concerns, such as differences in species, limited cell-cell interactions, and difficulty maintaining phenotype or cellular functions (Yin et al., 2015).
However, the usage of cortical organoids as a model for analysis can allow for new research avenues to form with proper design. Organoid models, which are artificially created masses of different cell types to resemble an actual organ, have been thoroughly researched in recent years due to their ability to consistently and efficiently recapitulate many aspects of the brain, such as the transcriptome, mutation type, and sensitivity to treatments (Velasco et al., 2016). Yet, the contributions of glial cells responsible for immune protection in the brain—the microglia—are poorly represented in these research models. The inclusion of microglia is necessary to see proper interactions with cancerous cells that resemble the actual brain organ environment. A few studies have looked into growing neural and microglia precursor cells and then putting them together, but this leads to poor gastrulation into the germ layers and inefficient microglia production within the organoid model (Popova et al., 2021). Furthermore, the effects of tumor fusion on organoids containing microglia have not been assessed, so the interactions between cancerous cells and microglia in cortical organoids are still obscure. This is important as microglia have two notable phenotypes that affect tumors in different ways: M1 and M2. M1 microglia are pro-inflammatory cells that release cytotoxic proteins to hinder tumor growth. Conversely, M2 microglia are anti-inflammatory cells that secrete certain proteins to promote tumor development. Understanding the state of microglia in the context of organoids can thus provide insights into interactions between the glial and cancerous cells.
To create cortical organoids, human neural progenitor cells (NPC) and human stem cell-derived microglia progenitor cells are co-cultured to form an organoid that better recapitulates the brain. The cells would mature together to become neurons and microglia through cell-cell interactions. The microglia precursor cells can be tagged with fluorescent protein for visualization. To investigate tumor fusion, resected clinical samples can be taken and then fused into the organoid early in development. Important tumor markers can be stained to track their growth as well. A few distinct genetic modifications could be performed in the middle of organoid development to see the effect of the gene changes on the tumor microenvironment. Moreover, mosaic analyses could be done to better understand the cell types that differentiate from a progenitor subtype (Yochem and Herman, 2005). This analysis essentially helps investigate gene function and cellular behavior in a subset of cells in a developing organism. The microglia within some organoids placed in each condition could be modified by introducing the gene cassette to the glial cells. This would be done to somehow alter the activity of microglia and ultimately interrogate molecular mechanisms underlying microglial response to tumor cells.
Eventually, after many tests and passing regulatory barriers, researchers can establish an ideal organoid containing a tumor microenvironment. This would allow for various drugs and therapeutic strategies to be implemented to test on the organoid as a platform. Moreover, the conditions of the microenvironment, driver mutations of the tumor, and phenotypes specific to the patient can be preserved and carefully controlled. Drug testing on organoids is also scalable and reproducible compared to other models, as the reproduction of organoids is inexpensive with minimal variation. Moreover, organoid production can be accelerated with technologies, like automatic microfluidic droplet platforms, to facilitate quicker clinical decisions after diagnosis (Ding et al., 2022).
Overall, cortical organoids could be excellent models for understanding the tumor microenvironment in the brain. Understanding the mechanisms underlying tumor growth in organoids can help scientists develop unique treatment approaches to save lives. Specifically, researchers can help create robust, reproducible, and scalable human brain tumor organoid models as a testing platform for making novel immune cell therapy strategies for detecting and eliminating brain cancer cells.
About the Author
Ishraq Haque is a rising junior at Harvard College, concentrating in Chemistry, with a secondary in Molecular and Cellular Biology.
References
However, the usage of cortical organoids as a model for analysis can allow for new research avenues to form with proper design. Organoid models, which are artificially created masses of different cell types to resemble an actual organ, have been thoroughly researched in recent years due to their ability to consistently and efficiently recapitulate many aspects of the brain, such as the transcriptome, mutation type, and sensitivity to treatments (Velasco et al., 2016). Yet, the contributions of glial cells responsible for immune protection in the brain—the microglia—are poorly represented in these research models. The inclusion of microglia is necessary to see proper interactions with cancerous cells that resemble the actual brain organ environment. A few studies have looked into growing neural and microglia precursor cells and then putting them together, but this leads to poor gastrulation into the germ layers and inefficient microglia production within the organoid model (Popova et al., 2021). Furthermore, the effects of tumor fusion on organoids containing microglia have not been assessed, so the interactions between cancerous cells and microglia in cortical organoids are still obscure. This is important as microglia have two notable phenotypes that affect tumors in different ways: M1 and M2. M1 microglia are pro-inflammatory cells that release cytotoxic proteins to hinder tumor growth. Conversely, M2 microglia are anti-inflammatory cells that secrete certain proteins to promote tumor development. Understanding the state of microglia in the context of organoids can thus provide insights into interactions between the glial and cancerous cells.
To create cortical organoids, human neural progenitor cells (NPC) and human stem cell-derived microglia progenitor cells are co-cultured to form an organoid that better recapitulates the brain. The cells would mature together to become neurons and microglia through cell-cell interactions. The microglia precursor cells can be tagged with fluorescent protein for visualization. To investigate tumor fusion, resected clinical samples can be taken and then fused into the organoid early in development. Important tumor markers can be stained to track their growth as well. A few distinct genetic modifications could be performed in the middle of organoid development to see the effect of the gene changes on the tumor microenvironment. Moreover, mosaic analyses could be done to better understand the cell types that differentiate from a progenitor subtype (Yochem and Herman, 2005). This analysis essentially helps investigate gene function and cellular behavior in a subset of cells in a developing organism. The microglia within some organoids placed in each condition could be modified by introducing the gene cassette to the glial cells. This would be done to somehow alter the activity of microglia and ultimately interrogate molecular mechanisms underlying microglial response to tumor cells.
Eventually, after many tests and passing regulatory barriers, researchers can establish an ideal organoid containing a tumor microenvironment. This would allow for various drugs and therapeutic strategies to be implemented to test on the organoid as a platform. Moreover, the conditions of the microenvironment, driver mutations of the tumor, and phenotypes specific to the patient can be preserved and carefully controlled. Drug testing on organoids is also scalable and reproducible compared to other models, as the reproduction of organoids is inexpensive with minimal variation. Moreover, organoid production can be accelerated with technologies, like automatic microfluidic droplet platforms, to facilitate quicker clinical decisions after diagnosis (Ding et al., 2022).
Overall, cortical organoids could be excellent models for understanding the tumor microenvironment in the brain. Understanding the mechanisms underlying tumor growth in organoids can help scientists develop unique treatment approaches to save lives. Specifically, researchers can help create robust, reproducible, and scalable human brain tumor organoid models as a testing platform for making novel immune cell therapy strategies for detecting and eliminating brain cancer cells.
About the Author
Ishraq Haque is a rising junior at Harvard College, concentrating in Chemistry, with a secondary in Molecular and Cellular Biology.
References
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- Fan, Y., Zhang, X., Gao, C., Jiang, S., Wu, H., Liu, Z., & Dou, T. (2022). Burden and trends of brain and central nervous system cancer from 1990 to 2019 at the Global, regional, and Country Levels. Archives of Public Health, 80(1).
- Popova, G., Soliman, S. S., Kim, C. N., Keefe, M. G., Hennick, K. M., Jain, S., Li, T., Tejera, D., Shin, D., Chhun, B. B., McGinnis, C. S., Speir, M., Gartner, Z. J., Mehta, S. B., Haeussler, M., Hengen, K. B., Ransohoff, R. R., Piao, X., & Nowakowski, T. J. (2021). Human microglia states are conserved across experimental models and regulate neural stem cell responses in chimeric organoids. Cell stem cell, 28(12), 2153–2166.e6.
- Velasco, S., Kedaigle, A. J., Simmons, S. K., Nash, A., Rocha, M., Quadrato, G., Paulsen, B., Nguyen, L., Adiconis, X., Regev, A., Levin, J. Z., & Arlotta, P. (2019). Individual brain organoids reproducibly form cell diversity of the human cerebral cortex. Nature, 570(7762), 523–527.
- Yin, X., Mead, B. E., Safaee, H., Langer, R., Karp, J. M., & Levy, O. (2016). Engineering stem cell organoids. Cell Stem Cell, 18(1), 25–38.
- Yochem, J., & Herman, R. K. (2005). Genetic mosaics. In WormBook: The Online Review of C. elegans Biology. WormBook.
