Brown Fat Exosomes In Therapy Of Obesity: Perspectives From The Mechanism And Imaging Study

Background:Obesity is an independent risk factor that severely threatens human life and health,and has become a serious public health challenge.Among the complications caused by obesity,fatty liver and cardiovascular disease are the common causes of death and disability associated with obesity,and they interact and influence each other.Obesity is closely related to adipose tissue.There are two types of adipose tissue in the body: white adipose tissue(WAT)and brown adipose tissue(BAT).Brown adipocytes contain many small lipid droplets and many mitochondria,and plays an important role in promoting energy metabolism and maintaining body temperature by inducing proton leakage of respiratory chain and thermogenesis via uncoupling protein 1(UCP1)located on the inner membrane of mitochondria.Brown fat can promote energy consumption,which makes it an ideal target for anti-obesity treatment and has attracted extensive attention.Several studies have shown that BAT transplantation can reduce body weights,lower blood glucose levels,and improve insulin sensitivity in obese mice.In addition,BAT has been reported to have cardiac protection effects.Although BAT transplantation has metabolic benefits and therapeutic effects on obesity,it is difficult to be used in clinical practice due to limited donor sources and immune intolerance.It is thus of great significance to find a way to improve obesity and metabolism without tissue transplantation.Exosomes,extracellular lipid vesicles secreted by cells with a diameter of 30-200 nm,have similar functions as their donor cells to a large extent,may serve as a potential alternative for cell therapy.Exosomes constitute an important way for intercellular communication.They carry and deliver bioactive molecules,including proteins and nucleic acids to recipient cells,regulating the biological functions of recipient cells.Compared with cell therapy,exosomes therapy has many advantages,including stability,immune tolerance,flexibility in drug administration,and high penetration through biological barriers such as blood-brain barrier and placenta barrier.Meanwhile,efficient drug loading and targeted delivery can be achieved through structural modification of exosomes.Exosomes have now been considered as promising alternatives for cell therapy.Therefore,to explore the function and key molecules of exosomes derived from BAT is expected to establish an alternative treatment for BAT transplantation.At present,there are few researches exploring the components and functions of BAT exosomes.What are the effects of BAT exosomes,especially in the context of obesity remain largely unknown.On the other hand,a variety of imaging technologies,including echocardiography and in vivo imaging system,provide us with effective means to understand the physiological and pathological process of diseases,explore the biological mechanisms,and evaluate the curative effects of different treatments.In the present study,we studied the effects of BAT derived exosomes on the obesity phenotype of mice induced by HFD,and preliminarily identified the potential mechanism for the regulatory roles of BAT exosomes on the basis of our previous experience in exosomes researches and the advantages of imaging platforms including small animal echocardiography.We have also explored strategies for improvement of exosomes therapy of obesity.Methods and results:1.BAT derived exosomes improve metabolism and cardiac function in obese miceWe established mice model of obesity by high fat diet(HFD)feeding.The weight of obese mice induced by HFD increased significantly,and their blood glucose level was also higher than that of control mice fed with normal chow diet(NCD).The systolic and diastolic functions of obese mice decreased compared with control mice.Histology examination showed that the myocardiocytes of obese mice exhibited slight hypertrophy and their liver had significant fat deposition.Hematological examination indicated that obese mice had deteriorated hepatic function and myocardial injury.The above results demonstrated that obese mice had metabolic disorders and decreased cardiac function.To study the role of BAT exosomes in obese mice,we extracted exosomes from interscapular BAT in 6-8 weeks healthy mice,and identified isolated exosomes by electron microscopy,particle size analysis and protein biomarker detection.We also isolated and identified serum exosomes to serve as control exosomes.After 6 weeks of treatment with exosomes,compared with mice in the control groups,mice received BAT exosomes treatment exhibited lowered body weights,improved glucose tolerance and decreased cholesterol level with reduced adiposity in SAT and VAT,alleviated liver injury and myocardial injury.Results from ultrasound evaluation showed that treatment with BAT exosomes could significantly improve cardiac function of obese mice.2.Brown fat exosomes function by promoting cell metabolismBy tracing the fluorescence labeled exosomes,we found that most of the injected exosomes accumulated in the liver and were taken by hepatocytes.Seahorse cell metabolism analysis showed that BAT exosomes can significantly improve the oxygen consumption rate of hepatocytes and promote cell metabolism.The results of protein mass spectrometry showed that the protein expression profiles of serum exosomes and BAT exosomes were significantly different.The results of gene ontology and enrichment analysis showed that most of the proteins enriched in BAT exosomes were mitochondria components and were involved in catabolism.The enrichment analysis of KEGG pathway showed that the protein enriched in BAT exosomes was extensively involved in fatty acid metabolism.3.Strategies for improvement of exosomes therapy of obesityIn order to facilitate the application of BAT exosomes in the treatment of obesity,we further explore the strategy improvements.Specifically,it involves improvement of exosomes targeting and the construction of induced BAT exosomes.We observed the effect of different injection routes on the distribution of exosomes by fluorescence labeling and mi RNA electro transfection.The results showed that the most exosomes injected via tail vein distributed in liver,spleen,and lung,while more exosomes by intraperitoneal injection distributed in subcutaneous adipose tissue(SAT)and visceral adipose tissue(VAT).The above results show that different targeting can be achieved by altering the administration route.Previous studies have shown that myoblasts can differentiate into brown adipocytes through reprogramming,which offers us the solution for BAT sources.We constructed vectors overexpressing PGC1α and PRDM16,the key regulatory genes of fat browning,and introduced them into mouse myoblasts to induce the browning of myoblasts and isolate induced exosomes to observe their effect on improving obesity phenotype.The results showed that similar to exosomes secreted by BAT,the exosomes secreted by the modified myoblasts C2C12 were capable of improving metabolism and cardiac function of obese mice to some extent.Conclusion:We report for the first time the effect of BAT exosomes on mice obesity phenotype and found that BAT exosomes can significantly improve the obesity phenotype of mice induced by HFD by promoting metabolism.Further investigation for alternative sources of BAT exosomes by means of cell engineering may become a new strategy for treatment of obesity.