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Using mini-brains to understand Zika virus-induced microcephaly

The human brain is notoriously hard to study. For centuries, anatomists tried to peer into the brain via autopsies. The latter is often not good enough as the subjects were dead and the diseases that the anatomists wanted to study were either in their late-stage or "contaminated" by various other comorbidities. The rise of genetics and molecular biology brought the use of animal models to prominence as animal brains tend to be "somewhat" similar to human brains. It is important the animal model brains are homologous (fancy word for similar) so that the findings could be applied in the clinic.

Often there are differences between animal model brains and human brains. For example, mice brains differ from human brains in terms of which cell types will later on in "their" life mature into neuronal cells. These differences make the inferences we get from studying animal models not always generalizable for certain types of conditions.

A bridge between studying dead human brains and studying non-human brains are the so-called cerebral organoids. Stemming from a Latin word "cerebrum" meaning "the brain", one could also call these molecular structures "mini-brains".

Cerebral organoids come from cells called undifferentiated stem cells. The latter cells are special in that they can become any tissue in the body - from bone to heart tissue. In order to become a specific cell type, these stem cells need a specific cocktail of sugars, proteins, and minerals. The secret "recipe" of what is needed for a stem cell to develop into a neuronal cell has been discovered fairly recently - in 2013. A stem cell has to have the necessary nutrients in the form of the precursor molecule cocktail described previously. The stem cells then need a special gel called Matrigel to simulate embryonic tissue coupled with a warm incubator that is set at body temperature. The last step in the recipe is adding a bit of spin inside the mixture to simulate blood flow. If left in these favorable conditions, stem cells will become 3-D structures that mimic the brain of a 10-week old fetus.

Are these brains sentient and do we do have to treat them like they are the equivalent to 10-week old fetus? That's a good ethical question that the scientists in the field are still thinking about. It seems that the general consensus is drifting towards treating cerebral organoids with the kind of respect that's given to animal models, but not necessarily fetuses of home sapiens.

Why? Well, cerebral organoids are not brains in that they don't have sensory input. They might show some intrinsic neural activity, but they do not perceive "light", "warmth", "touch" to the same extent that a potential fetus could. In other words, cerebral organoids are "not attached to a body" - that's why.

Scientists have already figured out how to derive undifferentiated stem cells from skin cells, so, one could derive a "personal" "mini-brain" for the patient. Crucially, cerebral organoids circumvent the inter-species differences that are hindering the applicability of animal model research. A very good example of where animal model research is not sufficient and where cerebral organoids are very useful in is the modelling of what happens in Zika virus-induced microcephaly ("micro" meaning small in Greek and "cephaly" coming from Greek word "kephale" - meaning head).

It has been reported that if a pregnant woman is bitten by a mosquito carrying the Zika virus, her child has a chance to develop microcephaly which leads to life-long developmental difficulties. What exactly is happening on a neurobiological basis has only been recently revealed by cerebral organoids. Multiple animal trials have tried to recapitulate the reduced head size observed in humans following infection by the Zika virus. These trials tried to genetically engineer mice to have recessive (in other words, inactive) versions of key neurodevelopmental genes that would lead to microcephaly. None of these trials worked and, thus, the story of what events take place in the brains of human babies during the disease could not have been recapitulated until 2013.

Scientists now understand that microcephaly is a result of a stunted differentiation (fancy word for maturing to adult neural cells) of neural progenitor cells that is caused by defects in cellular structures called cillium.

Cillium are known for being the motile pieces that clear our airways in the lungs. However, a lesser appreciated sub-group of cillium that are not motile is crucial in this problem. Non-motile cillium are responsible for controlling the cycle of the cell. The idea here is that each cell has its own "5-year plan" where it plans it out its growth, development and distribution of resources. In microcephaly, a gene disregulates normal functioning of the cillium and destroys any plans the cell had of normal development.

The modelling using cerebral organoids of microcephaly allowed scientists to understand that this dysregulation does not allow for "ready-to-be" neurons to become their adult selves and, thus, leads to underdeveloped brain sizes.

A cerebral organoid picture coloured using immunohistochemistry

Cerebral organoids sound like something out of a science fiction book, and really a spectacular advancement in neurodevelopmental studies. Sceptics of the technology caution that this model system has its own drawbacks. The most often cited is a stress response that the cells in these cerebral organoids are feeling. Nevertheless, this technology can act as a bridge between animal models, and indirect human studies like autopsies or neuroimaging. These so-called "mini-brains" can help us solve some of the most complex and difficult to understand neuropsychiatric disorders like schizophrenia and autism.

Cerebral organoids in a petri dish

The article was prepared by Matas Vitkauskas on behalf of INA

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