Multiscale Imaging Of Brown Adipose Tissue In Living Mice/Rats With Fluorescent Polymer Dots

Nowadays,the brown adipose tissue(BAT)has been identified as a promising target for the treatment of obesity,diabetes and relevant metabolism disorders due to the adaptive thermogenesis ability of this tissue.Visualizing BAT may provide an essential tool for pathology study,drug screening and efficacy evaluation.Currently,the most widely applied methods for detecting human BAT are PET/CT imaging and MRI.However,these methods require long scanning time and expensive instruments.Owing to limitations of current nuclear and magnetic resonance imaging approaches,it is of great importance to develop a method for high-throughput large-scale preclinical BAT imaging of small animals.Comparatively,fluorescence imaging has advantages in research on small animals imaging because it is highly sensitive,radical free with high imaging speed and low cost.Amang fluorescence imaging agents,polymer dots possess many ideal features such as good photostability,high brightness,facile synthesis and low toxicity.In this paper,we developed a BAT imaging method in living subjects based on a series of near-infrared polymer dots.Main research domains are as follow:(1)Design,synthesis and characterization of polymer dots.(2)Adipose stem cell imaging and labeling rate assessment.(3)Whole-body fluorescence imaging of BAT in mice and probe-based confocal laser endomicroscopy(pCLE)imaging of living mice / rats.(4)Histological analysis of tissue slices.Using the nano-precipitation method,we synthesized a series of MEH-PPV or PFBT based polymer dots.These polymer dots have different size and surface modification,and emit fluorescence in the near-infrared region by dopoing NIR-775.Thus,the fluorescence resonance energy transfer(FRET)system was established between conjugated polymers and NIR-775.In order to examine the BAT imaging efficacy between different polymer dots,we designed a series of longitudinal and multiscale experents from cells to tissues and to whole body levels.Using adipose stem cells we observed the uptake and labeling of polymer dots.Using pCLE imaging and light sheet imaging,we observed the real-time and still distribution in BAT relatdeted areas.We realized the whole-body BAT mapping by whole body imaging system and confimed these results through histology essays.As results,smaller sized polymer dots,MEH-PPV-COOH-30 and PFBT-COOH-30 showed distinct BAT imaging capacity.As early as 5 min after the intravenous injection of these polymer dots,extensive fluorescence was detected in the interscapular BAT(iBAT)and axillar BAT.In addition,axillar and inguinal white adipose tissues(WAT)were also recognized.The signal retio between iBAT and limb musle were above 2.5.The pCLE imaging recorded that the polymer dots could reach and scattered into the BAT in a real-time and still manner.Light sheet imaging confirmed the prefferd accumulation of polymer dots in BAT over WAT.Morover,these results were all further confirmed by histological essays.The imaging efficacy was verified at the cellular,histological and whole-body levels,and the whole study was conducted without a low temperature or pharmaceutical stimulation.Based on the present results,we believe that the fluorescent polymer dots could be a promising tool for the visualization of BAT in living subjects.