The blood–brain barrier (BBB) is composed of thin endothelial capillaries (red) wrapped by supporting pericytes (green) and astrocytes (yellow), enabling them to generate a tight barrier with highly selective transport functions for molecules entering the brain fluid from the blood stream. (Courtesy: Wyss Institute at Harvard University)
Researchers in the US have created a human blood–brain barrier (BBB) on a chip by recreating the development of brain microvascular capillaries in the embryo. The breakthrough could be used to develop more effective drugs and to better treat brain diseases (Nature Commun. 10.1038/s41467-019-10588-0).
The BBB’s primary primal role is to regulate access to the brain: it lets in the essential nutrients and energy metabolites required for good functioning of the brain while keeping toxins and pathogens at bay. Unfortunately, those substances that the brain staves off include potential life-saving drugs that could treat neurodegenerative disorders such as Alzheimer’s or Parkinson’s disease. For this reason, transient and localized opening of the BBB is one of the most researched and promising areas for emerging therapies that target the brain.
So far, studies have mainly resorted to animals such as mice or in vitro models to investigate the BBB and drug transport across it. But while these led to early breakthroughs in the field, they are not realistic enough to mimic the high functionality and complexity of the human barrier. Hence their use for development of drug and antibody shuttles that can cross the BBB is limited.
Culturing pluripotent stem cells under hypoxia
To solve this problem, Donald Ingber and his team from the Wyss Institute at Harvard University decided to upgrade one of their microfluidic organ-on-a-chip models of the BBB. Normally, the sealing of the BBB is assured by superposed layers composed of brain microvascular endothelial cells (BMVECs), adjacent multifunctional cells known as pericytes and non-neural brain cells called astrocytes. Instead of directly inserting adult cells in the two parallel channels of a chip, the team used induced pluripotent stem (iPS) cell technology and turned to physiology for inspiration.
Noticing that the BBB forms in the embryo with low-level oxygen (hypoxia), the researchers decided to culture the human iPS cells for an extended time in an atmosphere with only 5% oxygen concentration, instead of the normal 20%. Under these conditions, iPS cells developed into BMVECs that exhibited much more in vivo-like BBB properties than those grown under normal oxygen conditions.
These hypoxia-enhanced cells were subsequently cultured in one of the channels of a 2-channel microfluidic organ-on-a-chip device, while the other one was lined with human brain pericytes and astrocytes.
In vitro testing and drug development
The two channels in the device are separated by a porous membrane. The channel lined with BMVECs is irrigated with a blood-like fluid at a physiological level of shear stress, while the other channel is perfused with a fluid that mimics cerebral spinal fluid. After three days of microfluidic cultures, the BMVECs had formed a tight monolayer with high barrier properties and had established connections with surrounding pericytes and astrocytes through the pores in the membrane.
The human BBB recreated in vitro in this manner was two orders of magnitude tighter than those previously generated without hypoxia or fluid shear stress, or past culture models with endothelium derived from adult brain instead of iPS cells.
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| BMVECs form a microvessel by covering all four sides of the lower of two parallel microfluidic channels (lower images), while pericytes and astrocytes populating the upper channel connect to the microvessel across a dividing porous membrane. (Courtesy: Wyss Institute at Harvard University)
More importantly, the engineered BBB contained a higher number of the selective transport and drug shuttle systems that are known to be present in the human BBB in vivo. The team tested the chip by injecting drugs and substances used in the clinic to treat brain diseases. For example, an increasing concentration of a mannitol solute temporally opened the BBB to allow the passage of large drugs like the anti-cancer antibody cetuximab.
These experiments highlighted the ability for the hypoxia-enhanced human BBB chip to selectively either prevent drugs from reaching their targets in the brain or allow transport of nutrients and drugs across the BBB.
The chip could be used for drug development studies and to model aspects of brain diseases that affect the BBB such as Alzheimer’s and Parkinson’s disease. It also opens the gate to advanced personalized medicine approaches by using patient-derived iPS cells.
https://physicsworld.com/a/human-blood-brain-barrier-engineered-on-a-microchip/
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