Metabolic disorders are complex health problems, and with that complexity comes a necessity for dynamic models to study their causes and effects.
UC Berkeley researchers have designed an in vivo imaging system that can help them better study one metabolic disorder—fatty liver disease—in a real-time, non-invasive fashion. In vivo methods are those that can be performed in a living organism. While there have been ways to image fat uptake by other areas in the body, such as brown fat or the intestine, the location of the liver near many other fat-utilizing tissues has long posed a challenge.
When we eat a meal our blood gets an increase in fatty acids, the energy-rich molecules that are the building blocks of fats. Those fatty acids travel through the body’s circulation to many different tissues, including the liver, where they can be broken down and used for energy to perform the organ’s functions. However, when taken in at excessive levels, fatty acids can form deposits among the healthy tissue in the liver. This causes a condition called hepatosteatosis, or fatty liver disease.
In order to better understand hepatosteatosis, it is important to have a method for studying the fundamental mechanisms underlying lipid metabolism in the liver. In vivo imaging plays a major role in achieving that goal.
The imaging protocol is described in an article that appeared in the journal Gastroenterology in October. It builds on a method for fatty acid imaging already in use, but with several important alterations that heighten the clarity and accuracy of liver-specific images.
For these studies, laboratory mice are injected with a synthetic fat that is tagged with a molecule called luciferin. When these luciferin-fatty acid probes enter cells they release the luciferin, which in turn can produce light with the help of an enzyme from fireflies termed luciferase.
The innovation of first author Hyo Min Park, then a Ph.D. student in nutritional sciences and toxicology at UC Berkeley, came with breeding a new strain of mice that produce luciferase only in liver tissue. This adjustment ensures that the luminescence depicted in the images correlates to fat uptake solely by the liver.
“This system allows us a totally different approach for fatty acid flux studies,” said Hyo Min Park, who is now working for a biotech startup company he founded following graduation. He explains that current methods require sacrificing the animal in order to extract the liver and measure the amount of tagged fatty acids. For this reason, they are of limited value when studying a trend that requires monitoring across multiple time points in the same organism.
However, say that a group of researchers wants to measure the effect of a potential treatment over the course of one month. With Park’s method, they can measure fatty acid uptake every five or ten days in the same animals and track the effects during the entire period. The information garnered by continuous monitoring helps construct a more robust and detailed picture of cause and effect, one that is crucial during pre-clinical trials.
The article describes several findings made by Park and his colleagues using the new in vivo imaging method. In one 10-day trial, ingestion of fenofibrate, a medication currently used to treat high cholesterol and hyperlipidemia, resulted in a 40% increase in liver fatty acid uptake compared to control mice.
Now, that may seem counterproductive for shrinking the size of fat deposits in the liver - and that is because it is. Although fenofibrate has been known to increase the rate of fatty acid breakdown in the liver, the gross anatomy of the diseased livers would continue to appear pockmarked by fat deposits. The results from this trial indicate that this discrepancy could be partly explained by the observation that the drug itself causes increased fatty acid uptake to begin with.
“Previous studies demonstrated that fenofibrate increases beta oxidation and [fatty acid transporter] expression in the liver. But no one showed the effect of fenofibrate on hepatic fatty acid uptake increase in vivo,” Park explained. “After this experiment, I feel like I found a missing puzzle piece.”
In another trial performed by post-doctoral fellow Kim Russo and Lance Kriegsfeld, professor and vice chair of the UC Berkeley Department of Psychology, the in vivo system was used to monitor fatty acid uptake every hour for a 24-hr period. The results indicated that fatty acid uptake by the liver is altered across the day and night, suggesting a strong diurnal rhythm.
In addition to Park, Russo, and Kriegsfeld, study authors included Andreas Stahl, professor and chair of the Department of Nutritional Sciences and Toxicology. Michael Park, undergraduate student in Microbial Biology, aided with experiments. Luminescent fatty acid probes were produced by Grigory Karateev and Elena Dubikovskaya from the Swiss Federal Institute of Technology of Lausanne. This work was supported in part by the National Institutes of Health.