Why is this important?

Currently, our knowledge on brain development is hindered by limited access to relevant human tissues. Organoids capture development in a dish and therefore represent an ideal tool for its investigation in health and disease. Here, the researchers were able to co-culture macrophages with brain organoids driving the emergence of microglial traits and elucidating how these cells orchestrate neurogenesis in the embryo. On one hand, given broad efforts to establish more complex in vitro models, this study demonstrates that it is feasible to co-culture immune cells with organoids causing the acquisition of tissue-resident phenotypes. In addition, the authors gather evidence that the presence of microglia in the brain might promote the formation of blood vessels (or vasculogenesis), thus hinting at the possibility to incorporate in the model a further layer of complexity in the form of vasculature. On the other hand, the implications of the study go beyond bioengineering considerations. The authors show that microglia support NPCs with lipids during embryogenesis to modulate their differentiation, covering for astrocytes that are not yet present in the early brain (but would usually play this role). As discussed in this work, aberrations in microglia have already been correlated to increased likelihood of developing Alzheimer’s, and more could be uncovered on their role at the onset of neurodegenerative diseases using analogous models. Finally, validating their in vitro findings with in vivo data, the researchers corroborated the fidelity of their model as a proxy of the developing human brain. Such developments enhance the confidence of scientists in using organoid models for future brain research, making it an important stepping stone in brain research.

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How did the researchers do this?

The team generated a co-culture of brain organoids and iMac derived from the same iPSCs and compared it to brain organoids and iMac in isolation. They found that iMac penetrated the organoids and took on a microglia-like phenotype. They confirmed this by monitoring the spatial distribution of brain and immune cells, noting increased microglial markers on iMac, and using single-cell RNA sequencing (scRNA-seq) to show the upregulation of genes indicating the transformation of macrophages into iMicro. Additionally, they demonstrated improved neuronal function in the co-culture by studying the electrical activity of neurons using a technique known as patch-clamping. They also registered a reduction in size of the co-cultured brain organoids associated (i) to lower total cell numbers, and (ii) absolute and relative reduction in NPCs, which were less proliferative and were forming more frequent and long neurites (i.e., protrusions from the cell body that later become dendrites or axons) compared to the brain organoid-only controls. In other words, they gathered evidence that iMicro were limiting the proliferation of NPCs and favoring their differentiation into more mature neuronal cells. The researchers then asked themselves how iMicro were accomplishing this. Their scRNA-seq database had already hinted at an enhancement in lipid transport and storage in iMicro compared to iMac and a higher lipid content in co-cultured NPCs. Yet, NPCs were not synthesizing these lipids themselves. By tagging cholesterols with a tracker, researchers discovered that iMicro package them into droplets, which are then transferred to NPCs. Interestingly, when they used a drug to inhibit cholesterol transport, brain organoids not only grew larger but also showed an increase in NPC proliferation. Importantly, the team ultimately corroborated their findings by comparing them to publicly available datasets of mouse and human microglia at various developmental stages, in fact observing compatible trends.