A
graphical abstract of the gut-brain axis (left) and the 3D-printed in vitro
platform (right).
Courtesy: University of Maryland.
Anyone who
has ever experienced "butterflies in the stomach" before giving a big
presentation won't be surprised to learn there is an actual physical connection
between their gut and their brain. Neuroscientists and medical professionals
call this the "gut-brain-axis" (GBA). A better understanding of the
GBA could lead to treatments and cures for neurological mood disorders like
depression and anxiety, as well as for a range of chronic auto-immune
inflammatory diseases like irritable bowel syndrome (IBS) and rheumatoid
arthritis (RA).
The
problem is that up until now "butterflies" have been all medical
doctors have had to work with in detecting these GBA-related disorders. Even
today these diseases are primarily diagnosed by patients' own reports of their
symptoms. Finding "biomarkers"—objective measurements of a substance
in the body that indicates a disorder's presence—could dramatically improve
diagnosis and take an enormous burden off patients to correctly identify their
own symptoms.
Scientists
suspect the chemical neurotransmitter serotonin is the biomarker for a range of
GBA disorders. Serotonin spurs the nervous system into action via the vagus
nerve, the physical connector between the brain and the colon. Generated deep
within the lining of the gut, serotonin ultimately influences everything from
mood and emotions to sleep, digestion and the secretion of hormones. Its
production is in some way affected by the bacterial "microbiome"
present in this environment. Researchers hope that creating tools to analyze
serotonin's production and dysfunction in the gut microbiome will help unlock
the mysteries of GBA-related disorders.
With $1
million in National Science Foundation funding, University of Maryland (UMD)
engineers, neuroscientists, microbiologists and physicists have made
significant progress in developing a platform that can monitor and model the
real-time processing of gut microbiome serotonin activity. Their goal is to one
day package the platform into an ingestible capsule capable of detecting,
treating and monitoring GBA diseases.
Converging
disciplines is key, says Professor Reza Ghodssi, the project's principal
investigator. "We are converging neuroscience, molecular signaling, and
micro-nano devices and systems. This enables us to measure and investigate data
at the interface of each junction of a simulated GBA platform—cell to cell,
cell to molecule, molecule to nerve—and develop engineering methodologies to
analyze and interpret it."
The work
builds on ingestible medical device expertise that has been developed in the
UMD MEMS Sensors and Actuators Laboratory, the Fischell Department of
Bioengineering, and the Brain and Behavior Initiative.
Three new
published papers detail the progress in detecting serotonin, assessing its
neurological effects, and sensing minute changes to the gut epithelium.
In
"Electrochemical Measurement of Serotonin by Au-CNT Electrodes Fabricated
on Porous Cell Culture Membranes," the team developed a platform that
provides access to the specific site of serotonin production, important because
serotonin is secreted from the bottoms of cells. An innovative porous membrane
with an integrated serotonin sensor on which a model of the gut lining can be grown
allowed researchers to access both top and bottom sides of the cell culture.
The paper was published online Sept. 7, 2020 in the Nature journal, Microsystems and Nanoengineering. The authors are Bioengineering Ph.D. student Ashley Chapin, former ISR postdoctoral researcher Pradeep Ramiah Rajasekaran, alumnus David N. Quan (BioE Ph.D. 2015), Professor Liangbing Hu (MSE/MEII), Associate Professor Jens Herberholz (Psychology/NACS), Professor William Bentley (BioE/Fischell Institute/IBBR), and Professor Reza Ghodssi (ECE/ISR).
Courtesy:
University of Maryland
Using
metal deposition, they patterned a three-electrode system directly on a porous
cell culture membrane suspended within a custom 3-D-printed housing. Cells can
be grown on the top of the membrane with the serotonin sensor oriented on the
bottom for direct detection. The team then enhanced the sensitivity of
serotonin detection by increasing the electrode effective surface area,
drop-casting a small volume of carbon nanotubes on the electrode surface.
Prepared solutions of serotonin were detectable well within the expected
physiological concentration range.
The work
is the first to demonstrate a feasible method for detecting redox
molecules—such as serotonin—directly on a porous and flexible cell culture
substrate. It grants superior access to cell-released molecules and creates a
controllable model gut environment without resorting to invasive procedures on
humans or animals.
The team's
second paper, "A Hybrid Biomonitoring System for Gut-Neuron
Communication," builds on the findings of the first: the researchers
developed the serotonin measuring platform further so it could assess
serotonin's neurological effects. By adding and integrating a dissected
crayfish nerve model with the gut lining model, the team created a gut-neuron
interface that can electrophysiologically assess nerve response to the electrochemically
detected serotonin. This advance enables the study of molecular signaling
between gut and nerve cells, making possible real-time monitoring of both GBA
tissues for the first time.
The paper
was published online in the June 2020 IEEE Journal of Microelectromechanical
Systems. It was written by Chapin, Electrical and Computer Engineering Ph.D.
student Jinjing Han, Neuroscience and Cognitive Science Ph.D. student Ta-Wen
Ho, Herberholz and Ghodssi.
Finally,
the concept, design and use for the entire biomonitoring platform is described
in a third paper, "3-D Printed Electrochemical Sensor Integrated Transwell
Systems," published online Oct. 5, 2020 in the Nature journal Microsystems
and Nanoengineering. The paper was written by Rajasekaran, Chapin, Quan,
Herberholz, Bentley and Ghodssi.
This paper
delves into the development of the 3-D-printed housing, the maintenance of a
healthy lab-on-a-chip gut cell culture, and the evaluation of the two types of
sensors integrated on the cell culture membrane. The dual sensors are
particularly important because they provide feedback about multiple components
of the system—namely, the portions that model the gut lining's permeability (a
strong indicator of disease) and its serotonin release (a measure of communication
with the nervous system). Alongside the electrochemical sensor—evaluated using
a standard redox molecule ferrocene dimethanol—an impedance sensor was used to
monitor cell growth and coverage over the membrane. Using both these sensors
would allow monitoring of a gut cell culture under various environmental and
dietary conditions. It also would enable researchers to evaluate changes to
barrier permeability (a strong indicator of disease), and serotonin release (a
measure of communication with the nervous system).
"These
works represent a big step forward in our understanding of the gut/brain
axis," says Cornell University's John March, Chair of the Department of
Biological and Environmental Engineering. "One of the limitations of this
field is the inability to perform highly controlled experiments in a 'close to
in vivo' system. These papers provide ways around this problem with simple,
elegant experiments that are highly accessible. I expect these will be used
frequently."
Because
the engineering aspects of the platform are well underway, the researchers are
working towards culturing multi-tissue interfaces with the help of Jay Pasricha
and Subhash Kulkarni at Johns Hopkins University. Eventually multiple platforms
will be created, each colonized with a different combination of gut bacteria,
to measure the neurophysiological effects of serotonin production in varying
microbiome environments.
With this
information, Professor Wolfgang Losert (Physics/IPST/IREAP) will lead a machine
learning effort to process the sensor data through a computer model that can
simulate the outcomes of the different microbiomes. This will provide the
clearest picture yet of how a system as complex and individually unique as the
gut microbiome affects both gut and brain health. It also may help researchers
better understand the connection between nutrition and mental health.
"Understanding
biology at the level of whole organisms is a frontier in biology, and essential
to forming a basis for precision medicine," says the University of
California, Berkeley's Amy Herr, the John D. & Catherine T. MacArthur
Professor of Bioengineering. "By harnessing hallmarks of
engineering—integrated, systems-level design—the new research from the
Ghodssi-Bentley-Herberholz team presents an integrated approach to elegantly
perturb and then probe the electrons and molecules that are key conduits of
information flow in whole organisms."