Functional Imaging of Brown Fat in Mice with 18F-FDG micro-PET/CT

Summary

A method of functional imaging of mouse brown adipose tissue (BAT) is described in which cold-stimulated uptake of 18F-Fluorodeoxyglucose (FDG) in BAT is non-invasively assessed with a standardized micro-PET/CT protocol. This method is robust and sensitive to detect differences in BAT activities in mouse models.

Abstract

Brown adipose tissue (BAT) differs from white adipose tissue (WAT) by its discrete location and a brown-red color due to rich vascularization and high density of mitochondria. BAT plays a major role in energy expenditure and non-shivering thermogenesis in newborn mammals as well as the adults ¹. BAT-mediated thermogenesis is highly regulated by the sympathetic nervous system, predominantly via β adrenergic receptor ^{2, 3}. Recent studies have shown that BAT activities in human adults are negatively correlated with body mass index (BMI) and other diabetic parameters ^4-6. BAT has thus been proposed as a potential target for anti-obesity/anti-diabetes therapy focusing on modulation of energy balance ^6-8. While several cold challenge-based positron emission tomography (PET) methods are established for detecting human BAT ^9-13, there is essentially no standardized protocol for imaging and quantification of BAT in small animal models such as mice. Here we describe a robust PET/CT imaging method for functional assessment of BAT in mice. Briefly, adult C57BL/6J mice were cold treated under fasting conditions for a duration of 4 hours before they received one dose of ¹⁸F-Fluorodeoxyglucose (FDG). The mice were remained in the cold for one additional hour post FDG injection, and then scanned with a small animal-dedicated micro-PET/CT system. The acquired PET images were co-registered with the CT images for anatomical references and analyzed for FDG uptake in the interscapular BAT area to present BAT activity. This standardized cold-treatment and imaging protocol has been validated through testing BAT activities during pharmacological interventions, for example, the suppressed BAT activation by the treatment of β-adrenoceptor antagonist propranolol ^{14, 15}, or the enhanced BAT activation by β3 agonist BRL37344 ¹⁶. The method described here can be applied to screen for drugs/compounds that modulate BAT activity, or to identify genes/pathways that are involved in BAT development and regulation in various preclinical and basic studies.

Protocol

1. Animal Preparation and Cold Treatment

1. Locate and inspect a 4 °C cold room that has been approved for accommodating laboratory mice.
2. Pre-chill animal cages overnight in the cold room. The cages are assembled without feed and bedding but with a bottle of water.
3. In the morning of the experimental day, place mice one by one into each of the pre-chilled cages at 30 min intervals. Each singly caged mouse should stay in the cold room for nearly 4 hr before it is transported to the imaging lab. Ensure the mice are fasting but with access to water.
4. At 4 hr post cold treatment transport one animal at a time every ....

Results

An example of micro-PET/CT imaging of mouse BAT is shown in Figure 1. While the CT imaging provides anatomical information, the PET imaging encodes the distribution and quantity of ¹⁸F-FDG uptake throughout the whole body. These imaging data can be viewed separately ( Figure 1A and 1B), fused (Figure 1C), or demonstrated with a 3D feature such as maximal intensity projection (MIP, 1D). With the help of a 3D imaging tool, a volume of interest (VOI), her.......

Discussion

In this study a micro-PET/CT-based imaging method has been developed for detecting BAT activities in adult mice which simply requires a cold treatment and one injection of commercially available ¹⁸F-FDG. The whole procedure can be done in one day following a treatment and imaging sequence which starts every 30 minutes until all animals are treated and imaged. Under the experimental conditions outlined, a total of 10 mice (or 2 groups of 5 mice) can be tested on the same day with a single imaging system.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Micro-PET/CT Imaging System	Siemens Medical Solutions USA, Inc.	Inveon Dedicated PET System and Inveon Multimodality CT/SPECT System (docked)
Propranolol	Sigma	P0884
BRL 37344	Sigma	B169
18F-FDG	Cyclotope Inc.
C57BL/6J Male Mice	Jackson Laboratory	000664	3-4 months old