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.brown fat | mitochondria | ROS | UCP1 | electron transport chain U ncoupling protein 1 (UCP1) plays a role in acute adaptive thermogenesis in interscapular brown adipose tissue (BAT). UCP1 dissipates the mitochondrial protonmotive force (Δp) generated by the electron transport chain (ETC) and is important for thermal homeostasis in rodents and human infants (1, 2). Ucp1 orthologs are not limited to mammals, but are also expressed in ectothermic vertebrates (3) and protoendothermic mammals (4), suggesting that UCP1 may have an important role in biology beyond thermal control. For example, it is becoming increasingly evident that in specific respiratory states, UCP1 can reduce reactive oxygen species (ROS) levels in vitro (4-9). The mitochondrial ETC is a major source of ROS production in the cell, and ROS play important roles in physiology and pathophysiology (10-12). Reverse electron transport (RET) through mitochondrial complex I is a key mechanism by which ROS are generated in vivo (11, 13). Interestingly, RET relies critically on high Δp, whereas dissipation of Δp by UCP1 can lower ROS levels in isolated mitochondria (5-7).Thermogenic respiration in BAT is triggered by external stimuli that activate adrenergic signaling (14). Most notably, environmental cold induces the capacity for adrenergic-mediated BAT respiration in wild type (WT) animals, but only minimally in UCP1-KO animals (15, 16). It is understood that the respiratory response of BAT under these conditions is indicative of UCP1-mediated respiration; however, the rate of maximal chemically uncoupled oxygen consumption, an UCP1-independent parameter, is also lower in UCP1-KO adipocytes compared with WT (15, 16).Moreover, the basal respiratory rat...