Tiny brain, huge potential

NASHVILLE, Tenn.—The organ-on-a-chip concept is a relativelyrecent one in life-science research, but certainly nothing new, as researcherscreate tiny bioreactors to replicate human organs so they can see how humancells respond when exposed to minute quantities of toxins, disease organisms ornew drugs under development. But a multidisciplinary team lead by VanderbiltUniversity is now tackling one of the most complex, challenging and leastunderstood 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, theGordon A. Cain University Professor at Vanderbilt and director of VanderbiltInstitute for Integrative Biosystems Research and Education, and consisting ofresearchers from Vanderbilt University, Vanderbilt University Medical Center,the Cleveland Clinic and Vanderbilt's Nashville neighbor, Meharry MedicalCollege.

The grant is one of 17 that are being distributed by theNational Center for Advancing Translational Sciences at the U.S. NationalInstitutes of Health as part of its five-year, $70 million Tissue Chip for DrugTesting program, a cooperative effort that also involves the U.S. Food and DrugAdministration and the Defense Advanced Research Projects Agency (DARPA).

Organ-on-a-chip technology emulating the brain isparticularly important because the brain is protected by three barriers thatprevent intrusion by pathogens, but also block many therapeutic agents, themost formidable of these defenses being the blood-brain barrier (BBB). Inaddition, research has show that even when the brain's defenses don't stop acompound from getting through, they might alter its chemistry.

By replicating the kinds of chemical communication andmolecular traffic that occur in the human brain, the research team hopes thatthis 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 theCerebrovascular Research Center at the Cleveland Clinic, and his coworkers havedeveloped a hollow-fiber model of the BBB that uses two of the three cell typesthat make up the human barrier. Tests of this model reportedly have shown thatit accurately reproduces a number of the features of the real BBB.

But the desire to make as complete and accurate a model aspossible helped bring in other members of the team as well, such as Dr. Donald J.Alcendor, an associate professor of microbiology and immunology at MeharryMedical 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 vascularpericytes and the blood-brain barrier, I established a link with some investorswho knew about Prof. Wikswo and his brain-on-a-chip work, and I became veryinterested in joining in," Alcendor recalls. "I became especially interestedwhen I talked to Vanderbilt and Cleveland Clinic on the phone and realized thatpericytes weren't part of the model, and I think they are a vital part of theBBB that needs to be included in the microbrain model. Brain microvasculaturewith endothelial cells, astrocytes and pericytes will give us a much morecomplete picture."

Currently, the pharmaceutical industry employs a range of in-vitroscreening approaches to predict brain penetration of novel small moleculetherapeutics, including brain and plasma protein binding and immortalized celland membrane vesicle models that mimic the cell membrane-lipid bilayer orexpress cell surface drug transport proteins, notes Dr. J. Scott Daniels,director of drug metabolism and pharmacokinetics at Vanderbilt UniversityMedical Center and an assistant professor of pharmacology at the university.

Another approach the pharma and biotech industry employs isto measure brain penetration of small molecules in vivo in rodents.

"Still, each of these approaches too often fall short intheir clinical translational merit," he asserts. "The advent of the microbrainreactor not only would offer a human-cell derived blood-brain barrier withproperly expressed, functional drug transport proteins and drug metabolizingenzymes, but it would be malleable with respect to its ability to mimic humandisease states known to impact central nervous system (CNS) health and theintegrity of the BBB. One could envision a drug discovery organizationutilizing the technology to accurately predict the penetration of the CNS bynovel small-molecule drug candidates, predict development-limiting drug-druginteractions and even measure a pharmacological response as a result ofengaging a key receptor within the human brain microvasculature."

"What was not doable before was to have in a single modelboth neurons and blood vessels," Janigro says. "For example, brain cells canmetabolize drugs in an unpredictable manner. This new device allows us toassess what the brain does to the drug and what the drug does to the brain. Alimitation is the fact that it is likely that a given disease will havedifferent genetic and environmental backgrounds in different patients. The goalof a truly personalized medicine is yet to come."

Also potentially important, beyond just the neuroscienceapplications specifically, is the ability to better explore mechanical changesto cells, says Dr. John A. McLean of the Vanderbilt Institute of ChemicalBiology and Vanderbilt Institute of Integrative Biosystems, who is also anassistant professor of chemistry at the university. Sometimes, he notes,mechanical changes to cells can be as important or more so than biochemicalchanges.

One of the key challenges the team has been able to addressso far, Wikswo says, is the problem of needing to have the volume of the organsystem and each organ on a chip matched, lest one organ dominate the others orparacrine factors secreted by any organ be diluted below physiologicalthreshold by an unnaturally large perfusion volume.

This will be especially important for creatinghumans-on-a-chip, he says, given that in a "milliHuman," for example, theentire system can have only ~5 mL of perfusate.

In addition, Wikswo says, "The small volumes present cleardifficulties in both analytical chemistry and system control, which areaddressed by techniques that David Cliffel, John McLean and I have been workingon for years, which gives us a very clear path towards characterizing the dynamicsof drug responses in our microphysiological systems."

Wikswo notes that he is involved, for example, on work thatis funded through Harvard on the DARPA Microphysiological Systems Program todevelop a 10-organ microphysiological system, and another project through theLos Alamos National Laboratory to create a four-organ system on slightly largerscale.

But whether creating a human-on-a-chip, multi-organ chipmodels or a single organ model, the value is clear.

"Given the differences in cellular biology in the brains ofrodents and humans, development of a brain model that contains neurons and allthree barriers between blood, brain and cerebral spinal fluid, using entirelyhuman cells, will represent a fundamental advance in and of itself," Wikswosays.

That is almost certain to attract industry partners fordevelopment at some point, though there are no specific plans to commercializethe microbrain work.

"At present, though, I am searching for an industrialpartner to refine and commercialize our new microfluidic pump and valvetechnology that is critical to the perfusion controller/microclinical analyzersystems we are designing for all of these projects."