The contribution of mitochondrial Ca 2+ elevation to cold-stimulated thermogenesis is poorly understood. These components were known for decades but their molecular identities have been discovered only recently 19. Mitochondrial Ca 2+ uptake is mediated through a highly selective Ca 2+ channel, recently linked to the mitochondrial calcium uniporter (MCU) gene 15, 16, 17, Ca 2+ is subsequently transported out primarily by a mitochondrial Na +/Ca 2+ exchanger termed NCLX 18. Thereby, the fine-tuning of mitochondrial Ca 2+ homeostasis in conjugation with thermogenesis has to be tight and well regulated to avoid detrimental effects on cell physiology. The association between mitochondrial Ca 2+ overload and depolarization is not well understood, yet both stimuli were shown to be essential and sufficient for the pore opening, leading to mitochondrial swelling and triggering cytochrome c release, thus impairing respiration and culminating in cellular death 12, 13, 14. The opening of mPTP has been associated with two hallmarks, the first is depolarization and a collapse in membrane potential, which is accompanied by excess of mitochondrial Ca 2+ accumulation 8, 12. Yet, uncontrolled or prolonged mitochondrial Ca 2+ rise, often encountered during ischemic 8 or neurodegenerative diseases 9, can lead to opening of the mitochondrial permeability transition pore (mPTP) 10, 11. However, for the TCA cycle to keep up with the uniquely high energetic demand of thermogenesis, mitochondrial Ca 2+ is required for stimulating the activity of the dehydrogenases and ETC components 5, 6, 7. In addition to their role in inducing UCP1 function, FFAs undergo β-oxidation and fuel the TCA cycle for production of reducing equivalents, to supply the electron transport chain (ETC), essentially utilized to drive uncoupled energy expenditure 2. Resultant free-fatty acids (FFAs) activate BAT signature protein, uncoupling protein 1 (UCP1), leading to proton leak and uncoupled respiration 1, 3, 4. This pathway is initiated by the sympathetic neurotransmitter norepinephrine (NE), which induces a PKA-dependent lipid mobilization and oxidation. The brown adipose tissue (BAT) dissipates energy in the form of heat in response to adrenergic stimuli triggered by cold exposure 1, 2.

NCLX deletion transforms the adrenergic pathway responsible for thermogenesis activation into a death pathway. Our findings identify a key pathway through which BA evade apoptosis during adrenergic stimulation of uncoupling. Treatment with mPTP inhibitors rescue mitochondrial function and thermogenesis in NCLX-null BAT, while calcium overload persists. We show that Ca 2+ overload induced by adrenergic stimulation of NCLX-null BAT, triggers the mitochondrial permeability transition pore (mPTP) opening, leading to a remarkable mitochondrial swelling and cell death. Core body temperature, PET imaging of glucose uptake and VO 2 measurements confirm a thermogenic defect in NCLX-null mice. Adrenergic stimulation of NCLX-null brown adipocytes (BA) induces a profound mitochondrial Ca 2+ overload and impaired uncoupled respiration. Here, we show that adrenergic stimulation of BAT activates a PKA-dependent mitochondrial Ca 2+ extrusion via the mitochondrial Na +/Ca 2+ exchanger, NCLX. A sharp increase in mitochondrial Ca 2+ marks the activation of brown adipose tissue (BAT) thermogenesis, yet the mechanisms preventing Ca 2+ deleterious effects are poorly understood.

Ucsd software download kaleidagraph