Visualization and Quantification of Brown and Beige Adipose Tissues in Mice using [18F]FDG Micro-PET/MR Imaging

Summary

Functional imaging and quantitation of thermogenic adipose depots in mice using a micro-PET/MR imaging-based approach.

Abstract

Brown and beige adipocytes are now recognized as potential therapeutic targets for obesity and metabolic syndromes. Non-invasive molecular imaging methods are essential to provide critical insights into these thermogenic adipose depots. Here, the protocol presents a PET/MR imaging-based method to evaluate the activity of brown and beige adipocytes in mouse interscapular brown adipose tissue (iBAT) and inguinal subcutaneous white adipose tissue (iWAT). Visualization and quantification of the thermogenic adipose depots were achieved using [¹⁸F]FDG, the non-metabolizable glucose analog, as the radiotracer, when combined with the precise anatomical information provided by MR imaging. The PET/MR imaging was conducted 7 days after cold acclimation and quantitation of [¹⁸F]FDG signal in different adipose depots was conducted to assess the relative mobilization of thermogenic adipose tissues. Removal of iBAT substantially increased cold-evoked [¹⁸F]FDG uptake in iWAT of the mice.

Introduction

In response to changing nutritional needs, adipose tissue serves as an energy cache to adopt either lipid storage or mobilization mode to meet the needs of the body¹. Moreover, adipose tissue also plays a key function in thermoregulation, via a process called non-shivering thermogenesis, also called facultative thermogenesis. This is typically achieved by the brown adipose tissue (BAT), which expresses abundant level of mitochondria membrane protein uncoupling protein 1 (UCP1). As a proton carrier, UCP1 generates heat by uncoupling the proton transport and ATP production². Upon cold stimulation, thermogenesis in BAT is s....

Protocol

The protocol described below follows the animal care guidelines of The University of Hong Kong. The animals used in the study were 8-week-old C57BL/6J mice.

1. Animal surgical procedures and cold challenge

1. Perform interscapular BAT (iBAT) dissection.
   1. Anesthetize the mice by intraperitoneal injection of ketamine/xylazine (100 mg/kg bodyweight ketamine and 10 mg/kg bodyweight xylazine). After anesthesia, shave the hair of the mouse from the neck to just below the scapulae.
   2. Place the mice on the heating pad after disinfection and make a 2 cm incision along the dorsal midline of the mice.
   3. Remove the....

Results

Three groups of mice (n = 3 per group) underwent micro-PET/MR imaging in this study, where they were housed at either thermoneutrality (30 °C) or cold (6 °C) for 7 days. One group of mice (n = 3) had their iBAT removed (iBATx) prior to cold treatment (Figure 1A). This method led to an alteration to the white adipose tissue activity in all three mice. In particular, a remarkable increase in [¹⁸F]FDG uptake was observed in iWAT using micro-PET/MR imaging (

Discussion

In this study, a PET/MR -based imaging and quantification of functional brown and beige adipose tissue in small animal was described. This method uses the non-metabolizable glucose analog [¹⁸F]FDG as an imaging biomarker so as to identify the adipose tissues with high glucose-demand in a non-invasive manner. MR offers good soft tissue contrast and can better differentiate adipose fat tissues from the neighboring soft tissues and muscle. When combined with PET, this enables imaging and quantifying of the activa.......

Materials

Name	Company	Catalog Number	Comments
0.9% sterile saline	BBraun		0.9% sodium chloride intravenous infusion, 500 mL
5 mL syringe	Terumo	SS05L	5 mL syringe Luer Lock
Dose Calibrator	Biodex		Atomlab 500
Eye lubricant	Alcon Duratears		Sterile ocular lubricant ointment, 3.5 g
Insulin syringe	Terumo	10ME2913	1 mL insulin syringe with needle
InterView Fusion software	Mediso	Version 3.03	Post-processing and image analysis software
Isoflurane	Chanelle Pharma		Iso-Vet, inhalation anesthetic, 250 mL
Ketamine	Alfasan International B.V.	HK-37715	Ketamine 10% injection solution, 10 mL
Medical oxygen	Linde HKO	101-HR	compressed gas, 99.5% purity
Metacam	Boehringer Ingelheim		5 mg/mL Meloxicam solution for injection for dogs and cats, 10 mL
nanoScan PET/MR Scanner	Mediso		3 Tesla MR
Nucline nanoScan software	Mediso	Version 3.0	Scanner operating software
Wound clips	Reflex 7	203-100	7mm Stainless steel wound clips, 20 clips
Xylazine	Alfasan International B.V.	HK-56179	Xylazine 2% injection solution, 30 mL