Infinitely more complex than a mere wall, the blood-brain barrier controls the passage of most solutes that enter or leave the brain. To get a handle on how this molecular sieve influences neural activity, researchers led by Donald Ingber and Kevin Kit Parker at Harvard University modeled the interactions between blood vessels and brain tissue on a series of interconnected “organ chips.” These are small polymer surfaces etched with hollow channels in which cells can grow and fluids can flow. When connected through this blood-brain-barrier (BBB) model, endothelial cells, pericytes, astrocytes, and neurons alter their gene expression, they authors show. They also report a new type of metabolic coupling between the BBB and neurons—metabolites released from the vascular endothelium influence the production of key neurotransmitters. The findings were published August 20 in Nature Biotechnology.
“Parker’s multi-chip system has the potential to address several key topics of importance to AD, including Aβ clearance, the relationship between altered systemic metabolism and brain metabolism, the role of circulating factors in in neuronal inflammation and metabolism and vice versa, how neuronal factors might affect BBB physiology, and brain-derived biomarkers,” wrote Cheryl Wellington and Jérôme Robert of the University of British Columbia in Vancouver.
Joanna Wardlaw of the University of Edinburgh noted that while this technology might aid in studies of neurodegeneration, scientists have to be very cautious in their use and interpretation. “Cell behavior is influenced strongly by how cells interact with each other, so if something is missing the cells will behave differently,” she wrote.
Several in vitro models have been generated to study the BBB, including transwell cultures of endothelial and brain cells, artificial blood vessels, and, similar to this latest creation, microfluidic organ chips (Griep et al., 2013; Prabhakarpandian et al., 2013; Brown et al., 2015; Robert et al., 2017). These models have started integrating an ever-expanding menagerie of cells that live both in and around the barrier, from the endothelial cells that line vessel walls to the pericytes, astrocytes, and neurons in the brain. For most, the vessel and brain components cannot be disconnected, making it difficult to disentangle the contributions of different cell types to overall neurovascular unit (NVU) function.
Co-first authors Ben Maoz, Anna Herland, and Edward FitzGerald addressed this problem by modeling the NVU on three separate chips that could be studied when connected or individually. The three chips modelled flow from blood vessels into the brain, flow through the cerebrospinal fluid in the brain, and flow from the brain back out into the vessels. The BBB influx chip was lined with human brain microvascular endothelial cells (hBMVECs) on the bottom and sides to represent a brain blood vessel, while an upper chamber, separated by a porous membrane, contained a mixture of pericytes and astrocytes to mimic the perivascular space just outside of the vessel. The brain chip was populated with a mix of neurons and astrocytes to mimic the parenchyma. Lastly, the efflux chip contained the same cells as the influx BBB chip. When connected, the three chips modeled the flow of solutes from the blood into the brain, and out again (see image below).
In keeping with BBB function, the researchers found that the small, brain-permeant dye Cascade blue could pass between the chips, while larger molecules, such as antibodies, remained trapped in the first blood vessel chip. The researchers also validated the NVU system by injecting methamphetamine into the BBB influx chamber. As has been shown before, the stimulant transiently broke down the barrier, allowing antibodies to pass from the influx chip into the brain chip.
With these indications of a working BBB model, the researchers next assessed how coupling of the different compartments would affect the expression of proteins in each one. Using mass spectrometry, the researchers observed significant changes to the proteomes of each compartment when they linked them together. For example, in the perivascular compartment, fluidic coupling boosted levels of proteins involved in amino acid and protein biosynthesis. In all compartments, coupling reduced levels of proteins involved in migration and proliferation, but boosted metabolism-associated proteins.
The researchers next used mass spectrometry to study how metabolites secreted by cells in each compartment affected the NVU. In a nutshell, the metabolome of the BBB as a whole strongly swayed the activity of neurons in the brain chip. For example, they found that glutamate, a synaptic transmitter usually associated with neurons, could also be produced in endothelial cells, then transported into the brain chip. On the other hand, GABA, another neurotransmitter, was exclusively produced within the brain chip. However, synthesis of GABA in the brain compartment was strongly influenced by levels of glutamate and other metabolites transported across the endothelium.
“The finding that the endothelium can produce metabolic products that serve as substrates for neurotransmitters produced by brain cells raises the interesting possibility that one might be able to modulate brain function by targeting the brain endothelium,” wrote Ingber in an email to Alzforum. “The advantage here is that the drug would not have to cross the blood-brain barrier, which could be a significant plus.”
How might this system facilitate the study of neurodegenerative disease? Ingber said the approach could be particularly useful to study how factors outside of the brain, including immune cells, drugs, and toxins, might influence neurodegenerative processes. He added that the incorporation of patient-derived induced pluripotent stem cells (iPSCs) into the devices could lend itself to personalized medicine.
Per-Ola Freskgard of Roche in Basel called the study an important step forward in modeling the BBB in vitro. “Hopefully it means that we can stop using the simplistic transwell systems that have been around for decades, especially when studying more complex aspects of the human neurovascular unit,” he said. He pointed out that the three linked chambers proved useful in sorting out contributions from each compartment. “However, a system that brings the different functional parts closer, or into a signal unit, would potentially be more advantageous to recapitulate the in vivo architecture, but still have the opportunity to study the different compartments separately,” he added.
Costantino Iadecola of Weill Cornell Medical College in New York pointed out that the cellular composition and functional characteristics of the NVU vary substantially throughout the cerebrovascular tree, and that Parker’s model is best suited to model capillaries. “Overall, this is a step forward in capillary NVU modeling, which, owing to IPS cell technology, affords the opportunity of more faithfully recapitulating selected aspect of human diseases,” he wrote.
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Exosomes: Bubbling Over the Blood-Brain Barrier
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