By Alice de Bernardy
Currently, lumbar punction of cerebrospinal fluid (CSF) represents the main source of information we have about a brain’s health in vivo. While animal models have allowed for a vast amount of research to be conducted, they can be limiting in reflecting the level of complexity of the Human brain and can’t always model human neurodegenerative disorders.1 Hence, one of the main source of information remains from the study of post-mortem tissues, which raises evident limits concerning the understanding of diseases onset and development, as well as drug testing.
Cerebral organoids can counter some of those limitations. Organoids are 3D structures grown in vitro resembling the organization and development of organs found in vivo, and are a very promising technique at the frontline of today’s research.
Generation of a brain organoid can be let random[BK1] , by inducing neural differentiation onto induced pluripotent stem cells (iPSC).1 [BK2] The structures formed share many aspects found in Human, such as forebrain, midbrain, and hindbrain, as well as retina, choroid plexus and mesoderm. However, the organisation of each compartment remains quite random and spontaneous. As a result, methods have arised to build a brain organoid, piece by piece, by guiding the proliferation of iPSC into specific compartments of the brain. Chemical factors as well as non-biological scaffold can be used, the aim being to fuse specific organoid together to reach a more precise structure. For example, by fusing ventral and dorsal forebrain organoids, an assembloid is formed and both structures communicate and form microcircuits.1
Being able to grow a tiny brain in a dish can seem revolutionary for neurological research, and this technique has a very high potential. However, some limitations arise when studying these organ-resembling structure.
To begin with, brain organoids represent an early stage of development of the brain associated with early development in an embryo. For example, we cannot observe any folding on the surface of the organoid, since cortical folding only occurs around the 23rd week of pregnancy.1 Hence it makes the study of neurodegenerative disorders difficult as these arise mostly with age. Some methods have been found to speed up the differentiation process and induce ageing, for example by inhibiting NOTCH. But by interacting with a developing brain, the model obtained might not be quite as accurate of the reality in vivo.1 Another way of studying neurodegenerative diseases is to study the genetic forms of the condition. For example, by identifying a patient carrying the genetic condition linked to Parkinson’s disease. Researchers can isolate their cells, reset them to the state of iPSC and make a brain organoid out of them.
Overall, the early stage of development represented by the organoid can be seen as a technical complication, to consider when running experiment. However, the major limitation arises in the growth and maintenance of these tiny brains. By inducing neural differentiation, brain organoids are lacking vascularization, which is a major component for bringing enough oxygen to every parts of the brain.2 In comparison, the human brain is composed of about 650km of blood vessels .3 As a result, structures located in the center of the brain, such as NPC, which require high oxygenation, are lost. A necrotic core builds in the center, and the viable thickness is only restricted to about 400µm.1 Recent studies have managed to induce the development of a primitive vascular system formed of tubules and expressing markers of the Blood-brain barrier (BBB) and the blood-CSF-barrier (B-CSF-B), allowing free flow of the growth media. In both cases, expression of a vascular growth factor (VEGF) , or of ETS variant 2 (ETV2) has been shown to enhance neural differentiation, showing the importance of vascularizatIon in brain development, as those two events occur simultaneously in vivo.2,4,5 These methods enable improving brain organoid viability, but have not been proven to be very efficient on the long term, as the vasculature decreases after 4 months, probably due to the lack of blood pressure on the tubule.2
Despite some limitations, brain organoids offer a great insight into our human brains and deepens our understanding of diseases and drugs. For example, a recent study by Madeline Lancaster, pioneer of the field at the university of Cambridge found that Covid-19 affected our brain by inducing a leakage in the BBB and B-CSF-B at the choroid plexus.6 Brain organoids are therefore a great tool, which could become more and more relevant by combining new cutting-edge technologies in biotechnologies, stem cell research and neurology.
References:
1. Qian X, Song H, Ming G-L. Brain organoids: advances, applications and challenges. Development. 2019;146(8):dev166074.
2. Ham O, Jin YB, Kim J, Lee M-O. Blood vessel formation in cerebral organoids formed from human embryonic stem cells. Biochem Biophys Res Commun. 2020;521(1):84–90.
3. Mansour AA, Schafer ST, Gage FH. Cellular complexity in brain organoids: Current progress and unsolved issues. Semin Cell Dev Biol. 2021;111:32–9.
4. Cakir B, Xiang Y, Tanaka Y, Kural MH, Parent M, Kang Y-J, et al. Engineering of human brain organoids with a functional vascular-like system. Nat Methods. 2019;16(11):1169–75.
5. Shi Y, Sun L, Wang M, Liu J, Zhong S, Li R, et al. Vascularized human cortical organoids (vOrganoids) model cortical development in vivo. PLoS Biol. 2020;18(5):e3000705.
6. Pellegrini L, Albecka A, Mallery DL, Kellner MJ, Paul D, Carter AP, et al. SARS-CoV-2 infects the brain choroid plexus and disrupts the blood-CSF barrier in human brain organoids. Cell Stem Cell. 2020;27(6):951-961.e5.
Article written in June, 2022
Imperial Bioscience Review 