Spatiotemporally quantitative in vivo imaging of mitochondrial fatty acid β-oxidation at cellular-level resolution in mice.

Mitochondrial fatty acid β-oxidation (FAO) plays a key role in energy homeostasis. Several FAO evaluation methods are currently available, but they are not necessarily suitable for capturing the dynamics of FAO at a cellular-level spatial resolution and seconds-level time resolution. FAOBlue is a coumarin-based probe that undergoes β-oxidation to produce a fluorescent substrate, 7-hydroxycoumarin-3-(N-(2-hydroxyethyl))-carboxamide (7-HC). After confirming that 7-HC could be specifically detected using multiphoton microscopy at excitation/emission wavelength = 820/415-485 nm, wild-type C57BL/6 mice were randomly divided into control, pemafibrate, fasting (24 or 72 hours), and etomoxir groups. These mice received a single intravenous injection of FAOBlue. FAO activities in the liver of these mice were visualized using multiphoton microscopy at 4.2 seconds/frame. These approaches could visualize the difference in FAO activities between periportal and pericentral hepatocytes in the control, pemafibrate, and fasting groups. FAO velocity, which was expressed by the maximum slope of the fluorescence intensity curve, was accelerated in the pemafibrate and 72 hours fasting groups both in the periportal and the pericentral hepatocytes in comparison to the control group. Our approach revealed differences in the FAO activation mode by the two stimuli, pemafibrate and fasting, with pemafibrate accelerating the time of first detection of FAO-derived fluorescence. No increase in the fluorescence was observed in etomoxir-pretreated mice, confirming that FAOBlue specifically detected FAO . Thus, FAOBlue is useful for visualizing liver FAO dynamics at the single-cell level spatial resolution and seconds-level time resolution.