Copper imbalances are suspected to play a role in a number of diseases. But tools for detecting the metal in living organisms have been lacking. Christopher J. Chang, Andreas Stahl, and coworkers at the University of California, Berkeley, have now developed a bioluminescent probe for detecting copper in specific organs (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1613628113). Measurements with the probe reveal a copper deficiency associated with fatty liver disease, a condition in which fat accumulates in liver cells, leading to inflammation and potentially cirrhosis.
The probe is a caged luciferin molecule that is held in a nonluminescent state by a tris[(2-pyridyl)-methyl]amine ligand. When copper is present, oxidative cleavage of the ligand releases luciferin, which is then free to be a substrate for the enzyme luciferase and produce a bioluminescent signal. The magnitude of that signal is proportional to the copper concentration. The researchers achieved organ-specific copper detection by using genetically engineered mice that produce the signal-generating luciferase only in their livers.
The Berkeley team induced fatty liver disease in the mice by feeding them a high-fat diet. Measurements with the probe revealed that mice with fatty liver disease had less copper in their livers than did healthy mice.
“We identified that the copper deficiency shows up within about four weeks in the animal model,” Chang says. Other symptoms associated with fatty liver disease, such as glucose intolerance and insulin resistance, show up much later, he adds.
Earlier this year, Chang’s team reported that fat cells need copper to metabolize fat (Nat. Chem. Biol. 2016, DOI: 10.1038/nchembio.2098). “In one case, we showed that copper is essential for burning fat in fat cells,” Chang says. “Now we’re showing that if you accumulate a lot of fat in the wrong place, it’s accompanied by a deficiency in that same signaling metal.”
The new study “provides another puzzle piece on the hypothesis that nonalcoholic fatty liver disease is associated with copper deficiency,” says Peter Ferenci, a professor at the Medical University of Vienna who has been studying the connection between copper and fatty liver disease for 10 years. “The methods used are innovative and give a clear basis for further research on this topic.”
The probe molecule can’t be used for imaging in humans because of the need for luciferase, which was genetically engineered into the mice. But the Berkeley researchers plan to adapt the reaction-dependent copper detection for standard diagnostic methods such as MRI and PET imaging that can be used in humans. Chang is also eager to study whether copper supplementation could be used to treat fatty liver disease.