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Brain Organoids: Heads in a Jar
Early attempts to culture brain organoids yielded mixed results.
Futurama, 20th Century Fox Television


Lately, the scientific community has expressed a growing interest in a new take on an old discipline. Organoids, or the complex result of three-dimensional cell culture, were recently named the 2017 Nature Method of the year. While we have covered organoids before, we decided it was high time to revisit the topic in light of recent developments in the field.


For years, scientists have made incredible discoveries by observing how cells behave in two dimensions. Traditional cell culture techniques often rely on the propensity of cell lines or tissue samples to adhere and propagate on flat surfaces like glass or plastic. The ability to isolate tissue and grow it under carefully controlled conditions in the lab has led to game-changing advances, like the development and maintenance of genetically-stable, immortalized cell lines that serve as a benchmark for world-wide studies.
But as 3D printers and James Cameron proved in his 2009 sci-fi romp Avatar, sometimes 2D just isn't enough. Enter organoids: three-dimensional arrangements of multiple cell-types descended from a stem population, that resemble a particular model organ in composition and function. This basic definition hints at three important features of organoids:

  1. Much like their namesakes, organoids contain a similar mixture of cell types to those found in natural organs. Kidney organoids have been generated that contain as many as 10 distinct types of cells.


  2. Organoids can recapitulate organ function. Single Lgr5+ liver progenitor cells have been coaxed to form functional, transplantable organoids in vitro, with the potential to rescue liver disease related mortality in mouse models1.


  3. Organ-like cell grouping and spatial organization. Organoids self-organize (Get it?) by joining with cells expressing complimentary adhesion molecules (i.e. N-cadherin), in a process termed "cell sorting out." Cell sorting out is then followed by lineage commitment and arrangement into cohesive, 3D structures resembling natural tissue. The mechanisms behind this fascinating phenomenon remain poorly understood.

To date, organoid models have been developed for the liver, kidney, retina, breast, GI tract, and others. In this blog post, we'll explore recent advances in organoid models for what is perhaps the most complicated organ in the body: the brain.


Brain organoids are emerging as a much needed tool for studying brain development and neurodegenerative diseases. Diseases like Alzheimer's disease (AD) and Parkinson’s disease (PD) are difficult to model in mice due to the complex interaction of genetic and non-genetic risk factors that produce pathology. Practical and ethical considerations (anyone want to loan me a piece of their brain?) further limit the availability of healthy and diseased tissue for study. Organoids generated from patient-derived pluripotent stem cells (hPSC) have the potential to be important tools for understanding these disorders.


Earlier 3D culture methodologies using isolated neuroepithelium produced 2D neural tube-like arrangements of progenitors termed neural rosettes2. This method provided an important basis for ongoing work, but ultimately lacked the cytoarchitecture and cellular diversity of true organoids. Brain organoids more closely resembling human brain tissue were developed with a free-floating culture technique, allowing for the spatial polarization of cultured hPSC along rostral-caudal and dorsal-ventral axes using FGF, WNT, and BMP signal gradients3. Modifications to this technique by Lancaster et al., instead using hPSCs suspended in Matrigel® droplets cultured in a spinning bioreactor, produced "mini brains" complete with dorsal cortex, ventral forebrain, retina, hippocampus, and choroid plexus regions4. More recent publications detail cerebral organoids pieced together from independently grown regions, in order to increase reproducibility of cultured organoids5.


So, what is all this good for? Here are some recent examples of exciting applications for brain organoids:
  1. Zika virus research: Infection during pregnancy with the Zika virus is associated with severe birth defects in newborns (microcephaly), but little is known about the mechanisms driving these outcomes. Brain organoids are currently being used as a model system for the effects of viral infection on the developing brain, displaying many of the developmental abnormalities seen clinically. Dang et al. recently found that TLR3 signaling in Zika-infected cerebral organoids was linked to tissue shrinkage and apoptosis6.


  2. Models for neurodegenerative diseases: A modification of the neural rosette method, developed by Kim et al., utilized neural precursors suspended in Matrigel® to model the aggregation of β-amyloid and accumulation of hyperphosphorylated tau seen in AD7. More recently, Raja et al. took it a step further, using β- and γ-secretase inhibitor therapy on an organoid model of early-onset familial AD8.


  3. Autism research: Organoids generated from iPSC isolated from ASD patients serve as an invaluable model for early development of this condition. Studies have implicated FOXG1 and its downstream targets in the accelerated neural progenitor growth and accumulation of GABAergic inhibitory neurons observed in patient-derived organoids9.
While brain organoids continue to advance, they are still a long way off from faithfully replicating the dizzying complexity of the human brain. The lack of vascularization currently hinders organoid development in vitro, and the lack of immune cell populations limits their utility in fields like immuno-oncology. Despite these drawbacks, organoids represent a valuable research tool with exciting future potential. If you work with organoids and would like to know how BioLegend can help, email us at tech@biolegend.com.


References:
  1. Huch M, et al. 2013. Nature. 494(7436): 247-250
  2. Elkabetz Y, et al. 2008. Genes Dev. 22(2): 152-165
  3. Eiraku M, et al. 2008. Cell Stem Cell. 3(5): 519-32
  4. Lancaster MA, et al. 2013. Nature. 501(7467): 373-379
  5. Bagley JA, et al. 2017. Nat. Methods. 14: 743-751
  6. Dang J, et al. 2016. Cell Stem Cell. 19(2): 258-265
  7. Kim YH, et al. 2015. Nat. Protoc. 10(7): 985-1006
  8. Raja WK, et al. 2016. PLoS One. 11(9): e0161969
  9. Mariani J, et al. 2015. Cell. 162(2): 375-390
Contributed by Christopher Dougher, PhD.
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