UCLA

Proper regulation of cellular lipid storage and oxidation is indispensable for the maintenance of cellular energy homeostasis and health. Mitochondrial function has been shown to be a main determinant of functional lipid storage and oxidation, which is of particular interest for the adipose tissue as it is the main site of triacylglyceride storage in lipid droplets (LDs). Obesity stems from an imbalance between energy intake and energy expenditure, leading to increased lipid storage. Therefore, a better understanding of the mechanisms by which LD break-down and build-up are regulated can be crucial in the development of treatments against metabolic diseases.

Blocking mitochondrial pyruvate import in brown adipocytes induces energy wasting via lipid cyclingCombined fatty acid esterification and lipolysis, termed lipid cycling, is an ATP consuming process that contributes to energy expenditure. Therefore, interventions that stimulate energy expenditure through lipid cycling are of great interest. Here we find that pharmacological and genetic inhibition of the mitochondrial pyruvate carrier (MPC) in brown adipocytes activates lipid cycling and energy expenditure, even in the absence of adrenergic stimulation. We show that the resulting increase in ATP demand elevates mitochondrial respiration coupled to ATP synthesis and fueled by lipid oxidation. We identify that glutamine consumption and the Malate-Aspartate Shuttle are required for the increase in Energy Expenditure induced by MPC inhibition in Brown Adipocytes (MAShEEBA). We thus demonstrate that energy expenditure through enhanced lipid cycling can be activated in brown adipocytes by decreasing mitochondrial pyruvate availability. We here present a new mechanism to increase energy expenditure and fat oxidation in brown adipocytes, which does not require adrenergic stimulation of mitochondrial uncoupling.

The role of mitochondria attached to Lipid droplets in cellular energy and lipid metabolismRecent studies have identified a subpopulation of mitochondria attached to LDs, peridroplet mitochondria (PDM) that can be separated from cytoplasmic mitochondria (CM) by centrifugation. PDM have distinct bioenergetics, proteome, cristae organization and dynamics that support LD build-up. However, most of our knowledge on PDM function is based on PDM isolated from brown adipose tissue (BAT), mostly due to a lack of available protocols to isolate PDM from other tissues such as white adipose tissue (WAT). Here we show an optimized protocol to isolate PDM from BAT and WAT with high yield and purity by adding a proteolytic treatment. Using this protocol, we show that PDM attached to differently sized LDs have unique bioenergetics characteristics. Thus, our new isolation protocol has the potential to improve our understating of PDM function in WAT and BAT in different populations of LDs.