UCP1 deficiency causes brown fat respiratory chain depletion and sensitizes mitochondria to calcium overload-induced dysfunction

Significance We describe a physiological role for uncoupling protein 1 (UCP1) in the regulation of reactive oxygen species. Notably, the molecular differences between brown fat mitochondria from wild-type and UCP1 knockout (UCP1-KO) mice extend substantially beyond the deletion of UCP1 itself. Thus, caution must be taken when attributing a brown fat phenotype solely to UCP1 deletion when these animals are used. Given the wide utilization of the UCP1-KO mouse model, these data are of critical importance for the scientific communities studying obesity, thermogenesis and energy metabolism, and mitochondrial biology. Brown adipose tissue (BAT) mitochondria exhibit high oxidative capacity and abundant expression of both electron transport chain components and uncoupling protein 1 (UCP1). UCP1 dissipates the mitochondrial proton motive force (Δp) generated by the respiratory chain and increases thermogenesis. Here we find that in mice genetically lacking UCP1, cold-induced activation of metabolism triggers innate immune signaling and markers of cell death in BAT. Moreover, global proteomic analysis reveals that this cascade induced by UCP1 deletion is associated with a dramatic reduction in electron transport chain abundance. UCP1-deficient BAT mitochondria exhibit reduced mitochondrial calcium buffering capacity and are highly sensitive to mitochondrial permeability transition induced by reactive oxygen species (ROS) and calcium overload. This dysfunction depends on ROS production by reverse electron transport through mitochondrial complex I, and can be rescued by inhibition of electron transfer through complex I or pharmacologic depletion of ROS levels. Our findings indicate that the interscapular BAT of Ucp1 knockout mice exhibits mitochondrial disruptions that extend well beyond the deletion of UCP1 itself. This finding should be carefully considered when using this mouse model to examine the role of UCP1 in physiology.