Lipid droplets and mitochondria are anchored during brown adipocyte differentiation

Lipid droplets and mitochondria are anchored during brown adipocyte differentiation Dear Editor, Brown adipose tissue (BAT) acts as a site of non-shivering thermogenesis in mammals including adult humans (Nedergaard and Cannon, 2010).Prolonged cold exposure can 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 (Wu et al., 2013).Both mainly use mitochondrial β-oxidation of fatty acid (FA) from lipid droplets (LDs) in maintaining body temperature.The efficiency of hydrophobic FA transport between two organelles in aqueous cytoplasm has been challenged.Understanding the mechanism underlying the interaction between these two organelles is essential for animal physiology as well as for the development of therapies to treat metabolic diseases (Benador et al., 2018).Previously, we isolated LDs from BAT and found a tight contact between these two organelles (Yu et al., 2015).A recent study reported that LD-associated mitochondria can be separated from LDs by centrifugation at 9,000 ×g and termed them peridroplet mitochondria (PDM) (Benador et al., 2018).To reveal the nature of the interaction and avoid the impacts of housing temperature, we raised mice at their thermoneutral temperature (30 °C) and room temperature (23 °C), and analyzed their BAT.H&E staining demonstrated that most brown adipocytes from the 30 °C group were similar to white adipocytes with a unilocular LD (Fig. 1A, a andb) rather than multilocular LDs that are possessed in BAT at 23 °C (Fig. 1A, d ande).There was a clear change in mitochondrial morphology from an elongated form at 30 °C (Fig. 1A,c) to sphere-shaped at 23 °C (Fig. 1A,f).More importantly, physical contact between LDs and mitochondria was still detected in the 30 °C BAT (Fig. 1A,c, arrows) with no obvious differences in the amount of contact seen in the 23 °C BAT (Fig. 1A, f, arrows).BAT LDs were then isolated and stained with LipidTOX Green for LDs (Fig. 1B, a andd) and MitoTracker Red for mitochondria (Fig. 1B, b and e) using our previously established methods (Yu et al., 2015).The MitoTracker signals were detected on the LipidTOX-stained spherical structures in isolated LDs from both 30 °C and 23 °C BAT (Fig. 1B,c and f, arrows), further confirming that the contact between LDs and mitochondria exists in mouse BAT even persists at