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Tiny brain, huge potential
NASHVILLE, Tenn.—The organ-on-a-chip concept is a relatively recent one in life-science research, but certainly nothing new, as researchers create tiny bioreactors to replicate human organs so they can see how human cells respond when exposed to minute quantities of toxins, disease organisms or new drugs under development. But a multidisciplinary team lead by Vanderbilt University is now tackling one of the most complex, challenging and least understood organs in the body as they attempt to develop a microbrain.
Providing the financial foundation for this work is a new $2.1 million research grant awarded to the team led by Dr. John P. Wikswo, the Gordon A. Cain University Professor at Vanderbilt and director of Vanderbilt Institute for Integrative Biosystems Research and Education, and consisting of researchers from Vanderbilt University, Vanderbilt University Medical Center, the Cleveland Clinic and Vanderbilt's Nashville neighbor, Meharry Medical College.
The grant is one of 17 that are being distributed by the National Center for Advancing Translational Sciences at the U.S. National Institutes of Health as part of its five-year, $70 million Tissue Chip for Drug Testing program, a cooperative effort that also involves the U.S. Food and Drug Administration and the Defense Advanced Research Projects Agency (DARPA).
Organ-on-a-chip technology emulating the brain is particularly important because the brain is protected by three barriers that prevent intrusion by pathogens, but also block many therapeutic agents, the most formidable of these defenses being the blood-brain barrier (BBB). In addition, research has show that even when the brain's defenses don't stop a compound from getting through, they might alter its chemistry.
By replicating the kinds of chemical communication and molecular traffic that occur in the human brain, the research team hopes that this new type of brain model can offer insights into how the brain receives, modifies and reacts to various drugs and pathogens.
For example, Dr. Damir Janigro, director of the Cerebrovascular Research Center at the Cleveland Clinic, and his coworkers have developed a hollow-fiber model of the BBB that uses two of the three cell types that make up the human barrier. Tests of this model reportedly have shown that it accurately reproduces a number of the features of the real BBB.
But the desire to make as complete and accurate a model as possible helped bring in other members of the team as well, such as Dr. Donald J. Alcendor, an associate professor of microbiology and immunology at Meharry Medical College.
"I had a joint appointment at Vanderbilt in cancer biology, and as part of a lecture I was doing there for grad students on vascular pericytes and the blood-brain barrier, I established a link with some investors who knew about Prof. Wikswo and his brain-on-a-chip work, and I became very interested in joining in," Alcendor recalls. "I became especially interested when I talked to Vanderbilt and Cleveland Clinic on the phone and realized that pericytes weren't part of the model, and I think they are a vital part of the BBB that needs to be included in the microbrain model. Brain microvasculature with endothelial cells, astrocytes and pericytes will give us a much more complete picture."
Currently, the pharmaceutical industry employs a range of in-vitro screening approaches to predict brain penetration of novel small molecule therapeutics, including brain and plasma protein binding and immortalized cell and membrane vesicle models that mimic the cell membrane-lipid bilayer or express cell surface drug transport proteins, notes Dr. J. Scott Daniels, director of drug metabolism and pharmacokinetics at Vanderbilt University Medical Center and an assistant professor of pharmacology at the university.
Another approach the pharma and biotech industry employs is to measure brain penetration of small molecules in vivo in rodents.
"Still, each of these approaches too often fall short in their clinical translational merit," he asserts. "The advent of the microbrain reactor not only would offer a human-cell derived blood-brain barrier with properly expressed, functional drug transport proteins and drug metabolizing enzymes, but it would be malleable with respect to its ability to mimic human disease states known to impact central nervous system (CNS) health and the integrity of the BBB. One could envision a drug discovery organization utilizing the technology to accurately predict the penetration of the CNS by novel small-molecule drug candidates, predict development-limiting drug-drug interactions and even measure a pharmacological response as a result of engaging a key receptor within the human brain microvasculature."
"What was not doable before was to have in a single model both neurons and blood vessels," Janigro says. "For example, brain cells can metabolize drugs in an unpredictable manner. This new device allows us to assess what the brain does to the drug and what the drug does to the brain. A limitation is the fact that it is likely that a given disease will have different genetic and environmental backgrounds in different patients. The goal of a truly personalized medicine is yet to come."
Also potentially important, beyond just the neuroscience applications specifically, is the ability to better explore mechanical changes to cells, says Dr. John A. McLean of the Vanderbilt Institute of Chemical Biology and Vanderbilt Institute of Integrative Biosystems, who is also an assistant professor of chemistry at the university. Sometimes, he notes, mechanical changes to cells can be as important or more so than biochemical changes.
One of the key challenges the team has been able to address so far, Wikswo says, is the problem of needing to have the volume of the organ system and each organ on a chip matched, lest one organ dominate the others or paracrine factors secreted by any organ be diluted below physiological threshold by an unnaturally large perfusion volume.
This will be especially important for creating humans-on-a-chip, he says, given that in a "milliHuman," for example, the entire system can have only ~5 mL of perfusate.
In addition, Wikswo says, "The small volumes present clear difficulties in both analytical chemistry and system control, which are addressed by techniques that David Cliffel, John McLean and I have been working on for years, which gives us a very clear path towards characterizing the dynamics of drug responses in our microphysiological systems."
Wikswo notes that he is involved, for example, on work that is funded through Harvard on the DARPA Microphysiological Systems Program to develop a 10-organ microphysiological system, and another project through the Los Alamos National Laboratory to create a four-organ system on slightly larger scale.
But whether creating a human-on-a-chip, multi-organ chip models or a single organ model, the value is clear.
"Given the differences in cellular biology in the brains of rodents and humans, development of a brain model that contains neurons and all three barriers between blood, brain and cerebral spinal fluid, using entirely human cells, will represent a fundamental advance in and of itself," Wikswo says.
That is almost certain to attract industry partners for development at some point, though there are no specific plans to commercialize the microbrain work.
"At present, though, I am searching for an industrial partner to refine and commercialize our new microfluidic pump and valve technology that is critical to the perfusion controller/microclinical analyzer systems we are designing for all of these projects."