The "Browning Plasticity" Of White Adipose Tissue In Tree Shrews (Tupaia Belangeri) During Cold Acclimation And Rewarming

Many small mammals living in the environment of seasonal variation,different animals had different energy strategies to cope with the cold stress in winter.Adipose tissue plays an extremely important role in the regulation of animal energy homeostasis.Mammals have two main types of adipose tissue,white adipose tissue?WAT?and brown adipose tissue?BAT?.WAT contains large unicellular lipid vacuoles that store chemical energy in the form of fatty acids.BAT is characterized by multilocular lipid vacuole,a large number of mitochondria and high expression of uncoupling protein 1?UCP1?.BAT generates heat by burning lipids,which is a key heat-producing site in mammals.In recent decades,BAT has been considered to promote weight loss.Another type of brown adipoid cells is occasionally found in WAT in adult animals exposed to prolonged cold or exposure to stimulation of?-adrenergic factor?called beige or brite cells?.These induced beige cells have classic morphological and biochemical characteristics of BAT,including multilocular lipid droplets and UCP1 expression,a process known as WAT browning.In the process of adapting to lower temperature,the transformation of white fat cells into beige cells was significantly increased.Beige fat cells seem to be associated with reducing obesity or preventing diabetes in mice,and many data suggested that browning of WAT may reduce obesity.In the present study,tree shrews?Tupaia belangeri?was taken as the research object to study the changes of WAT under cold acculturation and rewarming by basic physiology,imaging,morphology,flow cytometry and gene expression,providing some theoretical basis for small mammals living in the seasonal changing environment.The present study was mainly divided into the following four parts:?1?Effects of cold acclimation and rewarming on body mass,food intake and rest metabolic rate?RMR?:There was no difference in body weight,food intake and RMR compared with the control group before the experiment.In the first 4 weeks,body weight,food intake and RMR of tree shrews increased significantly compared with the control group treated with low temperature.After rewarming,body mass in the experiment group decreased compared with that of the control group.In the process of adaptation,body mass,food intake and RMR had differences in the experimental group,but showed no significant differences in the control group.Body mass of the tree shrews increased with the extension of cold acclimation time,reached the maximum at cold acclimation?28 days?,and decreased in the rewarming process.These changes in body mass were associated with changes in RMR,food intake,and other biochemical indicators.Low temperature environment to stimulate the increase of energy consumption in tree shrews,by increasing body mass and RMR against the cold stress,through a lot of food intake to compensate for the energy consumption.It don't need too much energy consumption in tree shrews to adapt to a warm environment.?2?PET/CT scans?WAT mass and WAT morphology research under cold acclimation and rewarming:with ¹⁸F-FDG tag after anesthesia in tree shrews,and abdominal PET CT scan,to observe the abdomen absorption distribution profile of¹⁸F-FDG.The results showed that WAT absorbed the highest average radiation under cold acclimation?28 days?in tree shrews,average absorbed radiation decrease after rewarming and returned to control levels;HE staining sections showed:WAT had multicellular fat cells,more cells diameter smaller,they contain a large central lipid droplets and surrounded by several smaller lipid drops under cold acclimation;After rewarming,more multicellular cells disappear,and the more white fat cells diameter larger and appear more unilocular adipose cells and restored to the control level;Temperature had a significant effect on WAT mass of tree shrews.Low temperature induced WAT mass of tree shrews increased,and WAT mass decreased to the level of control group after rewarming.The results showed that the increase of abdominal heat induced by cold acclimation might be related to the browning of WAT.Morphologically,WAT showed the characteristics of brown fat during cold acclimation,showing that cold acclimation induced WAT browning.During browning,the tree shrews needed to increase WAT mass to make up for the energy consumed by adapting to low temperature.After rewarming,they all returned to the control level,reflecting the plasticity of WAT browning.?3?Gene expression studies under cold acclimation and rewarming:Rt-qPCR method is used to test peroxisome proliferation receptor??PPAR??,peroxisome proliferator-activated receptor?coactivator-1??PGC-1??,cyclooxygenase??COX??,bone morphogenetic protein 7?BMP7??PR domain containing 16?PRDM16?gene expression.The results showed that cold acclimation induced PPAR??PGC-1??COX??BMP7?PRDM16 gene expression increased significantly,the expression of these genes decreased to the level of the control group after rewarming.The above results showed cold acclimation induced browning of WAT and production of beige cells.When the temperature returned to warmer,no excessive energy consumption was required,so the gene expression associated with the browning of white adipocytes decreased,and the beige cells may be transformed into white cells,indicating that WAT browning is plastic.?4?Flow analysis under cold acclimation and rewarming of WAT in tree shrews:which came from three groups,the control group,cold acclimation group and the rewarming group,were labeled by fluorescent labeled UCP1 antibody and fluorescent labeled CD137 antibody,and then were analyzed by the flow cytometry to measure the white adipocytes which had UCP1 and CD137.As a result,the percentage of the white adipocytes which had UCP1 and CD137 in the cold acclimation group was higher than that in the control group significantly.Nevertheless,the percentage of the white adipocytes which had Ucp1 and CD137 in the rewarming group was lower than that in the cold acclimation group significantly and returned to the percentage of that in the control group.The increase of the white phagocytes which had UCP1 and CD137 was observed in the tree shrews during cold acclimation.The browning of WAT in tree shrews could be induced during cold acclimation.The plasticity of WAT browning was reflected by that the percentage of the white adipocytes which had UCP1 and CD137 returned to control levels after rewarming.In conclusion,the imaging,morphology,flow cytometry and gene expression levels had successively proved that the browning of WAT may have plasticity.Low temperature stimulates the increase of energy consumption.During cold acclimation,tree shrews produce beige cells to increase thermogenesis and cope with cold stress through WAT browning.During cold acclimation,white fat cells may transform into beige cells.After returning to a warm environment and no longer expend too much energy,beige fat cells may revert to mature white fat cells.In recent years,most studies have found that increasing BAT activity and promoting browning of WAT were strategies to fight obesity,and had increasingly become a key area of research on the treatment of obesity and related diseases.