Cerenkov luminescence imaging of interscapular brown adipose tissue

Imaging Brown Adipose Tissue with TSPO PET Tracers in Preclinical Animal Studies

Brown adipose tissue (BAT) is closely associated with thermogenesis and related to numerous diseases, including type 2 diabetes, nonalcoholic fatty liver disease (NAFLD), and obesity. Using molecular imaging technologies to monitor BAT could facilitate etiology elucidation, disease diagnosis, and therapeutics development. Translocator protein (TSPO), an 18 kDa protein that mainly locates on the outer mitochondrial membrane, has been proven as a promising biomarker for monitoring BAT mass. Here, we lay out the steps for imaging BAT with TSPO PET tracer [¹⁸F]-DPA in mouse studies.

Cerenkov luminescence imaging of interscapular brown adipose tissue using a TSPO-targeting PET probe in the UCP1 ThermoMouse

Rationale: [¹⁸F]fluorodeoxyglucose-positron emission tomography ([¹⁸F]FDG-PET) has been widely used as an imaging technique to measure interscapular brown adipose tissue (iBAT) activity. However, it is challenging to obtain iBAT-specific images using [¹⁸F]FDG-PET because increased uptake of [¹⁸F]FDG is observed in tumors, muscle, and inflamed tissues. Uncoupling protein 1 (UCP1) in the mitochondrial membrane, a well-known molecular marker of BAT, has been proposed as a useful BAT imaging marker. Recently, the UCP1 ThermoMouse was developed as a reporter mouse for monitoring UCP1 expression and investigating BAT activation. In addition, Translocator protein-18 kDa (TSPO) located in the outer mitochondrial membrane is also overexpressed in BAT, suggesting that TSPO-targeting PET has potential for iBAT imaging. However, there are no studies monitoring BAT using TSPO-targeting PET probes in the UCP1 ThermoMouse. Moreover, the non-invasive Cerenkov luminescence imaging (CLI) using Cerenkov radiation from the PET probe has been proposed as an alternative option for PET as it is less expensive and user-friendly. Therefore, we selected [¹⁸F]fm-PBR28-d₂ as a TSPO-targeting PET probe for iBAT imaging to evaluate the usefulness of CLI in the UCP1 ThermoMouse.

Methods: UCP1 ThermoMouse was used to monitor UCP1 expression. Western blotting and immunohistochemistry were performed to measure the level of protein expression. [¹⁸F]fm-PBR28-d₂ and [¹⁸F]FDG were used as radioactive probes for iBAT imaging. PET images were acquired with SimPET, and optical images were acquired with IVIS 100.

Results: UCP1 ThermoMouse showed that UCP1 and TSPO expressions were correlated in iBAT. In both PET and CLI, the TSPO-targeting probe [¹⁸F]fm-PBR28-d₂ was superior to [¹⁸F]FDG for acquiring iBAT images. The high molar activity of the probe was essential for CLI and PET imaging. We tested the feasibility of TSPO-targeting probe under cold exposure by imaging with TSPO-PET/CLI. Both signals of iBAT were clearly increased after cold stimulation. Under prolonged isoflurane anesthesia, TSPO-targeting images showed higher signals from iBAT in the short-term than in long-term groups.

Conclusion: We demonstrated that TSPO-PET/CLI reflected UCP1 expression in iBAT imaging better than [¹⁸F]FDG-PET/CLI under the various conditions. Considering convenience and cost, TSPO-CLI could be used as an alternative TSPO-PET technique for iBAT imaging.

Molecular Imaging of Brown Adipose Tissue Mass

Brown adipose tissue (BAT), a uniquely thermogenic tissue that plays an important role in metabolism and energy expenditure, has recently become a revived target in the fight against metabolic diseases, such as obesity, diabetes, and non-alcoholic fatty liver disease (NAFLD). Different from white adipose tissue (WAT), the brown adipocytes have distinctive features including multilocular lipid droplets, a large number of mitochondria, and a high expression of uncoupling protein-1 (UCP-1), as well as abundant capillarity. These histologic characteristics provide an opportunity to differentiate BAT from WAT using imaging modalities, such as PET/CT, SPECT/CT, MRI, NIRF and Ultrasound. However, most of the reported imaging methods were BAT activation dependent, and the imaging signals could be affected by many factors, including environmental temperatures and the states of the sympathetic nervous system. Accurate BAT mass detection methods that are independent of temperature and hormone levels have the capacity to track the development and changes of BAT throughout the lifetime of mammals, and such methods could be very useful for the investigation of potential BAT-related therapies. In this review, we focus on molecular imaging modalities that can detect and quantify BAT mass. In addition, their detection mechanism and limitations will be discussed as well.

Brown Adipose Tissue Rescues Bone Loss Induced by Cold Exposure

Cold temperature activates the sympathetic nervous system (SNS) to induce bone loss by altering bone remodeling. Brown adipose tissue (BAT) is influenced by the SNS in cold environments. Many studies have confirmed a positive relationship between BAT volume and bone mass, but the influence and mechanism of BAT on bone in vivo and in vitro is still unknown. Two-month-old C57/BL6j male mice were exposed to cold temperature (4°C) to induce BAT generation. BAT volume, bone remodeling and microstructure were assessed after 1 day, 14 days and 28 days of cold exposure. CTX-1, P1NP and IL-6 levels were detected in the serum by ELISA. To determine the effect of BAT on osteoclasts and osteoblasts in vitro, brown adipocyte conditional medium (BAT CM) was collected and added to the differentiation medium of bone marrow-derived macrophages (BMMs) and bone marrow mesenchymal stem cells (BMSCs). Micro-CT results showed that the bone volume fraction (BV/TV, %) significantly decreased after 14 days of exposure to cold temperature but recovered after 28 days. Double labeling and TRAP staining in vivo showed that bone remodeling was altered during cold exposure. BAT volume enlarged after 14 days of cold stimulation, and IL-6 increased. BAT CM promoted BMSC mineralization by increasing osteocalcin (Ocn), RUNX family transcription factor 2 (Runx2) and alkaline phosphatase (Alp) expression, while bone absorption was inhibited by BAT CM. In conclusion, restoration of bone volume after cold exposure may be attributed to enlarged BAT. BAT has a beneficial effect on bone mass by facilitating osteogenesis and suppressing osteoclastogenesis.

Magnetic Resonance Imaging Techniques for Brown Adipose Tissue Detection

Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) methods can non-invasively assess brown adipose tissue (BAT) structure and function. Recently, MRI and MRS have been proposed as a means to differentiate BAT from white adipose tissue (WAT) and to extract morphological and functional information on BAT inaccessible by other means. Specifically, proton MR (¹H) techniques, such as proton density fat fraction mapping, diffusion imaging, and intermolecular multiple quantum coherence imaging, have been employed to access BAT microstructure; MR thermometry, relaxometry, and MRI and MRS with ³¹P, ²H, ¹³C, and ¹²⁹Xe have shown to provide complementary information on BAT function. The purpose of the present review is to provide a comprehensive overview of MR imaging and spectroscopy techniques used to detect BAT in rodents and in humans. The present work discusses common challenges of current methods and provides an outlook on possible future directions of using MRI and MRS in BAT studies.

Multiscale Imaging of Brown Adipose Tissue in Living Mice/Rats with Fluorescent Polymer Dots

Brown adipose tissue (BAT) has been identified as a promising target for the treatment of obesity, diabetes, and relevant metabolism disorders because of the adaptive thermogenesis ability of this tissue. Visualizing BAT may provide an essential tool for pathology study, drug screening, and efficacy evaluation. Owing to limitations of current nuclear and magnetic resonance imaging approaches for BAT detection, fluorescence imaging has advantages in large-scale preclinical research on small animals. Here, fast BAT imaging in mice is conducted based on polymer dots as fluorescent probes. As early as 5 min after the intravenous injection of polymer dots, extensive fluorescence is detected in the interscapular BAT and axillar BAT. In addition, axillar and inguinal white adipose tissues (WAT) are recognized. The real-time in vivo behavior of polymer dots in rodents is monitored using the probe-based confocal laser endomicroscopy imaging, and the preferred accumulation in BAT over WAT is confirmed by histological assays. Moreover, the whole study is conducted without a low temperature or pharmaceutical stimulation. The imaging efficacy is verified at the cellular, histological, and whole-body levels, and the present results indicate that fluorescent polymer dots may be a promising tool for the visualization of BAT in living subjects.

Review of biomedical Čerenkov luminescence imaging applications

Čerenkov radiation is a fascinating optical signal, which has been exploited for unique diagnostic biological sensing and imaging, with significantly expanded use just in the last half decade. Čerenkov Luminescence Imaging (CLI) has desirable capabilities for niche applications, using specially designed measurement systems that report on radiation distributions, radiotracer and nanoparticle concentrations, and are directly applied to procedures such as medicine assessment, endoscopy, surgery, quality assurance and dosimetry. When compared to the other imaging tools such as PET and SPECT, CLI can have the key advantage of lower cost, higher throughput and lower imaging time. CLI can also provide imaging and dosimetry information from both radioisotopes and linear accelerator irradiation. The relatively short range of optical photon transport in tissue means that direct Čerenkov luminescence imaging is restricted to small animals or near surface human use. Use of Čerenkov-excitation for additional molecular probes, is now emerging as a key tool for biosensing or radiosensitization. This review evaluates these new improvements in CLI for both medical value and biological insight.