In vivo metabolic imaging and monitoring of brown and beige fat

Abstract

Activation of the thermogenic brown and beige fat is an effective means to increasing whole-body energy expenditure. In this work, a unique label-free method was developed to quantitatively assess the metabolism and thermogenesis of mouse adipose tissues in vivo. Specifically, an optical redox ratio (ORR) based on the endogenous fluorescence of mitochondrial metabolic coenzymes (nicotinamide adenine dinucleotide and flavin adenine dinucleotide) was used to measure the metabolic state of adipocytes. Our findings demonstrate that the ORR provides a label-free and real-time biomarker to determine the thermogenic response of brown, beige and white adipose tissues to a variety of physiological stimulations. In addition, the redox ratio also can be used to evaluate the degree of browning in the white fat of cold-acclimated mice. This technique is important to understand the recruitment and activation of thermogenic adipocytes in mammals and thus can help to develop therapeutic strategies against obesity.

In vivo study of metabolic dynamics and heterogeneity in brown and beige fat by label‐free multiphoton redox and fluorescence lifetime microscopy

“…Therefore, the optical redox ratio (ORR) was defined as the relative fluorescence intensity of NADH and FAD, and the oxidation-reduction ratio in mitochondria can be evaluated by the ORR using label-free fluorescence microscopy. We found that the ORR of BAT increases significantly after NE injection in mice, which is correlated with the temperature rise in BAT induced by thermogenesis [14]. In our recent work, the ORR was used to monitor the thermogenesis of BAT in vivo [14].…”

“…The SRS images and spectra suggest that the vacuoles structures in the fluorescence images of adipocytes were LDs enriched with triacylglycerol [14]. Since the lipids and cytoplasm of adipocytes have distinct fluorescence spectra [14], the lipid fluorescence can be excluded using dualchannel ratiometric image masks ( Figure S6). Since the lipids and cytoplasm of adipocytes have distinct fluorescence spectra [14], the lipid fluorescence can be excluded using dualchannel ratiometric image masks ( Figure S6).…”

“…Our previous study showed that the thermogenesis in BAT could be detected based on the ORR measured with a spectroscope system [14]. However, this nonimaging approach cannot be directly used for metabolic monitoring of beige fat, because it cannot differentiate the beige adipocytes from the neighboring white adipocytes.…”

“…Immunohistochemistry ( Figure 1C) and western blotting ( Figure 1D) also exhibit the generation of UCP1-abundant beige fat in the CL-316, 243-treated mice. The LDs in the adipocytes can also be visualized by their intrinsic fluorescence, which exhibits a similar spectrum to NADH fluorescence [14]. As shown in Figure 1A, the endogenous fluorescence signals of NADH and FAD in adipocytes were excited by an 810-nm femtosecond laser and were simultaneously collected by two separate detectors.…”

“…In the second process, NADH and FADH 2 are oxidized to generate NAD + and FAD via the electron transport chain (ETC) [8,25]. In our recent work, the ORR was used to monitor the thermogenesis of BAT in vivo [14]. Over the past decades, the ORR has been widely used as an intrinsic indicator of redox states in cells and tissues [26][27][28].…”