Mitochondrial dynamics regulate brown adiopcyte energy expenditure

Brown adipose tissue has gained much attention as a potential therapeutic approach for obesity and insulin resistance. This tissue is unique in its ability to go through hormonal mediated fast transition from efficient, ATP-generating metabolism to inefficient, heat producing metabolism. As such, the brown adipocyte is a unique system in which the relationship between mitochondrial architecture and bioenergetics can be studied.
Brown pre-adipocyes were harvested from C57BL6/J mice and differentiated in culture. Oxygen consumption (using Seahorse XF24) and mitochondrial membrane potential (using TMRE and Zeiss LSM 710 confocal microscope) were measured before and after activation with NE (1 uM) and FFA (Plamitate or Oleate 0.4 mM).

Immediately upon stimulation, mitochondrial networks underwent complete fragmentation and arrest of fusion. Fragmentation could only be achieved by beta3-adrenergic stimulation and could not be mimicked by treatment with free fatty acids. Examination of the mitochondrial fusion proteins revealed that the long form of Opa1 was cleaved during the activation. Remarkably, within each cell, mitochondrial membrane depolarization and mitochondrial fragmentation propagated in a wave-like fashion. To evaluate the role of mitochondrial fragmentation in energy expenditure, brown adipocytes were infected with a DRP1 dominant negative adenovirus. Inhibition of mitochondrial fission resulted in reduced levels of hormone stimulated oxygen consumption and membrane potential depolarization by 50%.
Together, these data reveals that mitochondrial fission is essential for the activation of uncoupled respiration in brown adipocytes and its transition from bioenergetic efficient to inefficient metabolism.