The current model used to study the human blood brain barrier has been discovered to have a serious flaw: one which casts previous research using the model into question.
The endothelial cells that line the blood vessels in the brain act as a barrier that prevents substances carried in the blood from entering the brain where they could potentially be dangerous, such as toxins or pathogens. Generally speaking, the only substances that pass through this barrier are water, some gasses, and lipid-soluble substances, although other compounds such as glucose can be actively transported into the brain.
Studying the blood brain barrier in humans is difficult and it is not possible to use animal models as they differ considerably from humans. The easiest way to conduct research is to use a model of the blood brain barrier in the lab. One such model was developed in 2012 which involved taking differentiated human cells and converting them into induced pluripotent stem cells (IPSCs) – stem cells that behave in a similar way to embryonic stem cells. The IPSCs are then differentiated into endothelial cells, such as those that line the blood vessels in the brain and spinal cord.
Now researchers at Columbia University Vagelos College of Physicians and Surgeons and Weill Cornell Medicine have discovered a serious problem with this model – The endothelial cells generated from ISPCs were found to behave differently to brain microvascular endothelial cells. That discovery led the researchers to believe that the method of generating the endothelial cells used in the blood brain barrier model resulted in endothelial cells being generated with the wrong identity.
The researchers reproduced the protocol for developing the model and subjected the cells to bulk single-cell RNA sequencing. They discovered that the cells in the model lacked several proteins found in natural brain microvascular endothelial cells, and instead more closely resembled endothelial cells found elsewhere in the body, but not in the brain.
The researchers identified three genes that could be activated in IPSCs which would result in endothelial cells being generated that behaved more like endothelial cells in the blood vessels in the brain, although the researchers still need to do more work to create a reliable and accurate model of the human blood brain barrier.
“The misidentification of human brain endothelial cells may be an issue for other types of cells made from induced pluripotent cells such as astrocytes or pericytes that form the neurovascular unit,” said Dritan Agalliu, PhD, Co-Study Leader, Associate Professor, Pathology and Cell Biology, Vagelos College of Physicians and Surgeons, Columbia University. “The protocols to generate these cells were created before the advent of single-cell technologies that are better at uncovering a cell’s identity.”
You can read more about the research in the paper – Pluripotent stem cell-derived epithelium misidentified as brain microvascular endothelium requires ETS factors to acquire vascular fate – which was recently published in Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.2016950118.