In a study published today in Scientific Reports, scientists describe a noninvasive method for imaging the progression of atherosclerotic plaques in living mice. Atherosclerosis is a disease involving the deposition of fatty material on the walls of blood vessels, and it is a key player in many cardiovascular diseases.
The University of Tsukuba research team induced atherosclerosis in mice by removing a receptor involved in transporting cholesterol into cells and then feeding the mice a high-cholesterol diet. To image the plaque, the team replaced the native immune cells with immune cells that were genetically engineered to express a fluorescent protein.
"A main advantage of our approach is that the introduced immune cells, as macrophages, congregate in atherosclerotic plaque,” says senior author Yoshihiro Miwa, “so the level of fluorescence emitted by them strongly correlates with the amount of plaque that has formed.”
While this technique is great in theory, in reality, bioluminescent proteins and visible light fluorescent proteins have poor deep-tissue penetration and high autofluorescence. This makes simple, noninvasive detection of the macrophage plaques without injecting imaging reagents very difficult. Yoshihiro and colleagues incorporated a new near-infrared fluorescent protein, iRFP, into the genetically engineered immune cells, allowing for strong deep-tissue penetration with minimal background autofluorescence.
To detect whether this new, noninvasive imaging technique could be used to monitor the progression of atherosclerotic plaques, the team monitored the imaging over time in mouse aorta, and they found that the detected signal increased gradually with time in a manner that correlated with plaque progression.
"Because we can now clearly analyze the amount of plaque present and its change over time, our work should lead to more effective monitoring of how well anti-atherosclerotic drugs work," says corresponding author Michito Hamada. "This method can also reduce the number of experimental animals used because there's no need to sacrifice them and remove tissues for analysis at each time point within an experiment."
Image: IVIS image of the thoracic area in HCD-fed control and iRFP mice. Image courtesy of the University of Tsukuba.