Interaction Between Lipid Droplets And Mitochondria In Brown Adipocytes

Brown adipose tissue(BAT)functions as a site of non-shivering thermogenesis in mammals.It is abundant in newborns and in hibernating mammals,and also found in adult humans and other animals.Prolonged cold exposure can also induce the acquisition of thermogenic function in white adipose tissue(WAT)with the cells that have undergone the "browning" process being referred to as beige cells.BAT and beige cells oxidize fatty acid(FA)stored as triacylglycerol(TAG)to produce heat.Two organelles with specific relevance to the thermogenic function are lipid droplets(LDs)and mitochondria.The LD is a membrane-bound organelle that contains a neutral lipid core and a phospholipid monolayer with a complement of proteins.Nearly all organisms,including prokaryotes,contain the organelle.In animal cells,the LD is a storage site for TAG,and plays important roles in metabolic homeostasis.Mitochondria,at other hand,couple the oxidation of reduced carbon substrates to the production of ATP in all cells and can altern atively produce heat in BAT and beige cells.The primary source of energy for thermogenesis is(3-oxidation of LD-derived FA.The processes of TAG hydrolysis in LDs and FA ?-oxidation in mitochondria are crucial to the maintenance of BAT thermogenesis.The FA has to be transported into mitochondria where the ?-oxidation occurs,and the efficiency of this transfer is essential for thermogenesis.The aqueous phase of cytosol is not suitable for hydrophobic FA transport.Therefore,whether FA is direct or indirect transfer between the two organelles raises great interests in the field.Recently,several omics studies have provided new insights into how LDs in BAT respond to temperature change.However,these studies used mice living at room temperature(22?-23?)that are already in an activation state for BAT since the thermoneutral temperature for mice is about 30?.In addition,most other previous studies were also carried out at room temperature,thus missing the activation process of BAT between 30? and 23?.Therefore,we raised two sets of mouse populations,each with a group of 4 mice,at three different temperatures:low temperature(6?),medium temperature(23?),and thermoneutral temperature(30?),collected their BATs,and compared their status under these conditions.To study the two key organelles and their responses to cold treatment,we isolated BAT LDs and cytosolic mitochondria(CM)from these mice,and then analyzed their proteomes comparatively.First,we found that the association between LDs and mitochondria exists in animals raised at 30?,suggesting that the formation of this connection is not induced by cold exposure.The interaction between LDs and mitochondria was not disrupted by general physical or chemical methods in the laboratory and was not altered across the temperature groups.In addition,LD-anchored mitochondria(LDAM)contained electron transport chain(ETC)complexes with normal activities.Based on the comparative proteomic analysis,a subset of the proteins in these organelles were differentially regulated in response to environmental temperature,including those involved in the glycolysis,gluco/glycero/neogenesis,FA-oxidation,amino acid metabolism,and oxidative phosphorylation pathways.It was also found that LDAM had a higher level of proteins related to uncoupling and were more responsive to cold than cytosolic mitochondria(CM).These results suggest that the LDs and LDAM are a preorganized organelle complex permitting a rapid response to cold challenge.To identify the interaction mechanism,we differentiated the brown adipocyte precursors into brown adipocytes and observe the behaves of these two organelles in the process.During the differentiation,the mitochondria started to interact with LDs at 2nd day and reached to saturation at 4th day,suggesting that this contact is a native property of brown adipocytes.This tight contact was also detected previously in other oxidative tissues such as skeletal muscle and heart,but not in liver of mice.Importantly,this contact was also found in the skeletal muscle and heart,but not in liver of Rhesus monkeys.Our study proposed that the tight connection allows the two organelles to coordinate the regulation of their functions and permits the transfer of hydrophobic FA without through the hydrophilic environment.Our new method and study also pave a way and provide several clues to dissect the molecular mechanism of BAT activation and how LDs interact with the mitochondria.