A team of researchers from the Massachusetts General Hospital (MGH) Center for Engineering in Medicine (MGH-CEM) have created a "liver on a chip," a model of liver tissue that replicates the metabolic variations found throughout the organ and more accurately reflects the distinctive patterns of liver damage caused by exposure to environmental toxins, including pharmaceutical overdose. Their report has been published online in the journal Scientific Reports.
"Our goal with this project was to create a liver tissue construct that responds to toxins the same way the liver in your body does," says William McCarty, PhD, a postdoctoral fellow at MGH-CEM and the paper's lead author. "The liver is a chemical processing plant, but it's not a single vat; different locations within the liver react differently to drugs and toxins. Here, we exploited microfluidics to control the metabolism of liver cells down to a resolution of a few cells, allowing us to create liver tissue that shows the same patterns of toxicity caused by differences in drug metabolism as the liver in your body."
When blood passes through the liver, it travels from arteries to veins through channels called sinusoids, lined with the liver cells called hepatocytes. From one end of the sinusoid to another, the hepatocytes have different metabolic functions, often controlled by external factors and gene expression. For example, the cells closest to the arterial end of the sinusoid are most efficient at releasing glucose that has been stored in the form of glycogen, while cells at the venous end are most efficient at taking up and storing glucose. Similar differences for other liver functions are well known, with metabolic changes occurring across the 25-cell length of the sinusoid.
In order to develop a system that more closely replicates the metabolic differences among hepatocytes, the research team developed a microfluidic device that distributes hormones or other chemical agents across a 20- to 40-cell-wide sample of hepatocytes in such a way that the effects on the liver cells vary from one side to the other. For example, if blood-sugar-lowering insulin is fed into one of the device's two inlets while glucagon, which raises blood sugar, is added through the other, the metabolism of the hepatocytes is changed so that those on one side release glucose while those on the other take it up. The use of other agents produced similar results across the field of hepatocytes regarding nitrogen metabolism or alcohol degradation, and use of a molecule that induces the expression of drug metabolism enzymes resulted in varied zones of susceptibility to the toxic effects of acetaminophen.
"Investigators have been developing in-vitro liver models for 40 years, but all of those systems ignore the distinct patterns of metabolically active hepatocytes that exist within the liver sinusoid" says Martin Yarmush, MD, PhD, director of the MGH-CEM and the paper's senior author. "We hope this tool, which displays zonation of carbohydrate and nitrogen metabolism, in addition to drug detoxification and alcohol degradation, will improve our ability to understand and predict the effects of toxins and new drugs on the liver."
Co-author Berk Usta, PhD, of MGH-CEM, adds, "While further replication and validation using more compounds are needed, this study demonstrates the importance of fine control of liver metabolism. Since many liver pathologies also show regional variation, this tool may also serve as a basis for models of liver disease." Berk and Yarmush are both members of the Harvard Medical School faculty. The work in this study was supported by National Institutes of Health grants UH2TR000503, F32DK098905 and 1R21EB020192-01.
Massachusetts General Hospital, founded in 1811, is the original and largest teaching hospital of Harvard Medical School. The MGH Research Institute conducts the largest hospital-based research program in the nation, with an annual research budget of more than $800 million and major research centers in HIV/AIDS, cardiovascular research, cancer, computational and integrative biology, cutaneous biology, human genetics, medical imaging, neurodegenerative disorders, regenerative medicine, reproductive biology, systems biology, photomedicine and transplantation biology. The MGH topped the 2015 Nature Index list of health care organizations publishing in leading scientific journals, earned the prestigious 2015 Foster G. McGaw Prize for Excellence in Community Service and returned to the number one spot on the 2015-16 U.S. News & World Report list of "America's Best Hospitals."